More accurate fix for bug #25140

ext/mbstring/mbstring.c

@@ -1864,7 +1864,7 @@ MBSTRING_API char * php_mb_convert_encoding(char *input, size_t length, char *_t
 	if (output_len) {
 		*output_len = 0;
 	}
-	if ( !input || !length) {
+	if (!input) {
 		return NULL;
 	}
 	/* new encoding */
@@ -2004,7 +2004,7 @@ PHP_FUNCTION(mb_convert_encoding)
 	if (ret != NULL) {
 		RETVAL_STRINGL(ret, size, 0);		/* the string is already strdup()'ed */
 	} else {
-		RETVAL_STRINGL("", 0, 1);
+		RETVAL_FALSE;
 	}
 	if ( s_free) {
 		efree(s_free);