cmd/goyacc: use a constant instead of zero value

Found with honnef.co/go/tools/cmd/unused.

Change-Id: I373d710e0f5f98dcf591826a26541d1d12696e14
Reviewed-on: https://go-review.googlesource.com/38175
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed