ww source: reject malformed UTF-8

cmd/wcc/ww.h

@@ -203,6 +203,7 @@ struct Lex {
 	Arena	*a;		/* token-text arena */
 	int	errs;
 	u64	nulcount;	/* raw NUL bytes diagnosed by the source decoder */
+	u64	utf8count;	/* malformed UTF-8 bytes diagnosed/filtered */
 	int	modreset;	/* a `//ww:module-reset` directive was seen in
 				 * the last skipped run; lexnext emits TK_MODRESET
 				 * before the next real token. */