rc: avoid single-byte overflow in lexer
 [rsc] --rw-rw-r-- M 467740 glenda sys 6102 Mar 12 09:15 sys/src/cmd/rc/lex.c
	/n/sourcesdump/2006/0312/plan9/sys/src/cmd/rc/lex.c:149,155 - 
	/n/sourcesdump/2006/0313/plan9/sys/src/cmd/rc/lex.c:149,155
	  {
	  	if(p==0)
	  		return 0;
	- 	if(p==&tok[NTOK]){
	+ 	if(p==&tok[NTOK-1]){
	  		*p = 0;
	  		yyerror("token buffer too short");
	  		return 0;