fix php#9535

Author: NathanFreeman

ext/mbstring/libmbfl/mbfl/mbfilter.c

@@ -1097,6 +1097,7 @@ mbfl_strcut(
 		mbfl_convert_filter *encoder     = NULL;
 		mbfl_convert_filter *decoder     = NULL;
 		const unsigned char *p, *q, *r;
+		int original_pos = 0;
 		struct {
 			mbfl_convert_filter encoder;
 			mbfl_convert_filter decoder;
@@ -1276,7 +1277,11 @@ mbfl_strcut(
 			bk = _bk;
 		}
 
-		(*encoder->filter_flush)(encoder);
+		if (device.pos != 0) {
+			original_pos = device.pos;
+			(*encoder->filter_flush)(encoder);
+			device.pos = original_pos;
+		}
 
 		if (bk.decoder.filter_dtor)
 			bk.decoder.filter_dtor(&bk.decoder);

ext/mbstring/tests/gh9535.phpt

@@ -0,0 +1,16 @@
+--TEST--
+GH-9535 (mb_strcut(): The behavior of mb_strcut in mbstring has been changed in PHP8.1)
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+$input = 'あaいb';
+$bytes_length = 10;
+$encoding = "ISO-2022-JP";
+$converted_str = mb_convert_encoding($input, $encoding, mb_internal_encoding());
+$cut_str = mb_strcut($converted_str, 0, $bytes_length, $encoding);
+$reconverted_str = mb_convert_encoding($cut_str, mb_internal_encoding(), $encoding);
+var_dump($reconverted_str);
+?>
+--EXPECT--
+string(4) "あa"