Ref-head partial object doc rules can cause nil pointer dereference in compiler

[johanfylling]

The following module will cause a nil pointer dereference at

opa/ast/index.go

Line 498 in 35b07b5

node.values.Add(rule.Head.Value)

in the BuildRuleIndices compiler stage:

package test
import future.keywords

p.q contains "foo"

p[q] := r if {
	q := "bar"
	r := "baz"
}

Omitting the if keyword causes the issue to not be triggered.