Panic when sending "null" to the state endpoint

[mjhcorporate]

Description of the bug

Hoverfly panics when sending null to /api/v2/state.

Steps to reproduce the issue

Start hoverfly, then

curl --request PUT \
  --url http://localhost:8601/api/v2/state \
  --header 'content-type: application/json' \
  --data 'null'

Observed result

Hoverfly error messages seen (If none, say none)

PANIC: runtime error: invalid memory address or nil pointer dereference
goroutine 7 [running]:
github.com/codegangsta/negroni.(*Recovery).ServeHTTP.func1()
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/recovery.go:159 +0xad
panic({0xce9080?, 0x159f390?})
        /usr/local/go/src/runtime/panic.go:770 +0x132
github.com/SpectoLabs/hoverfly/core/handlers/v2.(*StateHandler).Put(0xc000114be0, {0x7f19b860eaf8, 0xc00011c030}, 0xc0003cc6c0, 0x0?)
        /home/circleci/hoverfly/core/handlers/v2/state_handler.go:73 +0xf3
github.com/codegangsta/negroni.HandlerFunc.ServeHTTP(0x10?, {0x7f19b860eaf8?, 0xc00011c030?}, 0x467da5?, 0x10?)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:29 +0x2d
github.com/codegangsta/negroni.middleware.ServeHTTP({{0xfa9560?, 0xc000115210?}, 0xc0000113c8?}, {0x7f19b860eaf8, 0xc00011c030}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:38 +0xa8
github.com/SpectoLabs/hoverfly/core/handlers.(*AuthHandler).RequireTokenAuthentication(0x20?, {0x7f19b860eaf8?, 0xc00011c030?}, 0x0?, 0x0?)
        /home/circleci/hoverfly/core/handlers/auth_handler.go:50 +0x153
github.com/codegangsta/negroni.HandlerFunc.ServeHTTP(0xc00047b800?, {0x7f19b860eaf8?, 0xc00011c030?}, 0xd75660?, 0x412101?)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:29 +0x2d
github.com/codegangsta/negroni.middleware.ServeHTTP({{0xfa9560?, 0xc000115200?}, 0xc0000113b0?}, {0x7f19b860eaf8, 0xc00011c030}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:38 +0xa8
github.com/codegangsta/negroni.(*Negroni).ServeHTTP(0xc0002e30e0, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:96 +0x105
github.com/go-zoo/bone.(*Route).parse(0xc0000c8c60, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/go-zoo/bone/route.go:171 +0x1ba
github.com/go-zoo/bone.(*Mux).parse(0xc0002c2000, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/go-zoo/bone/helper.go:23 +0x91
github.com/go-zoo/bone.(*Mux).DefaultServe(0xc0002c2000, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/go-zoo/bone/bone.go:71 +0x2b
github.com/go-zoo/bone.(*Mux).ServeHTTP(0xc000580008?, {0x7f19b860eaf8?, 0xc00011c028?}, 0x0?)
        /home/circleci/hoverfly/vendor/github.com/go-zoo/bone/bone.go:90 +0x98
github.com/codegangsta/negroni.(*Negroni).UseHandler.Wrap.func1({0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0, 0xc000091380)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:46 +0x45
github.com/codegangsta/negroni.HandlerFunc.ServeHTTP(0x40ec27?, {0x7f19b860eaf8?, 0xc00011c028?}, 0x8?, 0x3?)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:29 +0x2d
github.com/codegangsta/negroni.middleware.ServeHTTP({{0xfa9560?, 0xc000010318?}, 0xc000010720?}, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:38 +0xa8
github.com/codegangsta/negroni.(*Recovery).ServeHTTP(0xc00047bb00?, {0x7f19b860eaf8?, 0xc00011c028?}, 0xd75660?, 0x412101?)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/recovery.go:193 +0x7b
github.com/codegangsta/negroni.middleware.ServeHTTP({{0xfa9460?, 0xc0000a2e10?}, 0xc000010330?}, {0x7f19b860eaf8, 0xc00011c028}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:38 +0xa8
github.com/codegangsta/negroni.(*Negroni).ServeHTTP(0xc0002e3710, {0xfae720, 0xc000194380}, 0xc0003cc6c0)
        /home/circleci/hoverfly/vendor/github.com/codegangsta/negroni/negroni.go:96 +0x105
net/http.serverHandler.ServeHTTP({0xfac678?}, {0xfae720?, 0xc000194380?}, 0x6?)
        /usr/local/go/src/net/http/server.go:3142 +0x8e
net/http.(*conn).serve(0xc000314480, {0xfafe10, 0xc000333a10})
        /usr/local/go/src/net/http/server.go:2044 +0x5e8
created by net/http.(*Server).Serve in goroutine 1
        /usr/local/go/src/net/http/server.go:3290 +0x4b4

Expected result

State should set to the "default" state, i.e. all state cleared.

Additional relevant information

1. Hoverfly version: v1.10.7

[tommysitu]

Thanks for the report @mjhcorporate I've raised a PR to fix it.