goyacc: fix indexing bug when yydebug >= 2 Fixes issue 2701.
goyacc: fix indexing bug when yydebug >= 2
Fixes issue 2701.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5556070

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go