5g: correct size of reg array.

Found by gcc 4.5.2 -Werror build reported on IRC by niemeyer.

R=ken2, rsc, r2
CC=golang-dev
https://golang.org/cl/4438042

src/cmd/5g/gg.h

@@ -52,7 +52,7 @@ struct	Prog
 
 EXTERN	Biobuf*	bout;
 EXTERN	int32	dynloc;
-EXTERN	uchar	reg[REGALLOC_FMAX];
+EXTERN	uchar	reg[REGALLOC_FMAX+1];
 EXTERN	int32	pcloc;		// instruction counter
 EXTERN	Strlit	emptystring;
 extern	char*	anames[];