v1.2.0 enters infinite loop

Running `errcheck` on the CockroachDB code.

With errcheck v1.1.0:
```
$ time ~/roach2/bin/errcheck -exclude pkg/testutils/lint/testdata/errcheck_excludes.txt ./pkg/...

real	0m7.966s
user	0m45.962s
sys	0m5.611s
```

With v1.2.0, the same command doesn't finish (I waited 10 minutes). I also notice that a single core is pinned at 100% while it's running.