ww source: reject raw NUL characters

cmd/wcc/ww.h

@@ -202,6 +202,7 @@ struct Lex {
 	i32	col;
 	Arena	*a;		/* token-text arena */
 	int	errs;
+	u64	nulcount;	/* raw NUL bytes diagnosed by the source decoder */
 	int	modreset;	/* a `//ww:module-reset` directive was seen in
 				 * the last skipped run; lexnext emits TK_MODRESET
 				 * before the next real token. */