cmd/yacc: error rather than panic when TEMPSIZE is too small

cmd/yacc: error rather than panic when TEMPSIZE is too small

I tried simply increasing the size of the slice but then I got an error
because NSTATES was too small. Leaving a real fix for after 1.7.

Update #16144.

Change-Id: I8676772cb79845dd4ca1619977d4d54a2ce6de59
Reviewed-on: https://go-review.googlesource.com/24321
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>