Move out of bound offset into the switch error code handler

Author: Girgias

ext/mbstring/mbstring.c

@@ -2100,17 +2100,6 @@ PHP_FUNCTION(mb_strpos)
 		RETURN_FALSE;
 	}
 
-	if (offset != 0) {
-		size_t slen = mbfl_strlen(&haystack);
-		if (offset < 0) {
-			offset += slen;
-		}
-		if (offset < 0 || offset > slen) {
-			zend_value_error("Offset not contained in string");
-			RETURN_THROWS();
-		}
-	}
-
 	n = mbfl_strpos(&haystack, &needle, offset, reverse);
 	if (!mbfl_is_error(n)) {
 		RETVAL_LONG(n);
@@ -2127,6 +2116,9 @@ PHP_FUNCTION(mb_strpos)
 		case 8:
 			zend_value_error("Argument is empty");
 			RETURN_THROWS();
+		case 16:
+			zend_value_error("Offset not contained in string");
+			RETURN_THROWS();
 		default:
 			zend_value_error("Unknown error in mb_strpos");
 			RETURN_THROWS();