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>

src/cmd/yacc/yacc.go

@@ -683,6 +683,10 @@ outer:
 		levprd[nprod] = 0
 	}
 
+	if TEMPSIZE < ntokens+nnonter+1 {
+		errorf("too many tokens (%d) or non-terminals (%d)", ntokens, nnonter)
+	}
+
 	//
 	// end of all rules
 	// dump out the prefix code