Fix fseek undefined behavior

Author: arshidkv12

main/streams/memory.c

@@ -128,7 +128,7 @@ static int php_stream_memory_seek(php_stream *stream, zend_off_t offset, int whe
 	switch(whence) {
 		case SEEK_CUR:
 			if (offset < 0) {
-				if (ms->fpos < (size_t)(-offset)) {
+				if (ms->fpos < -(size_t)offset) {
 					ms->fpos = 0;
 					*newoffs = -1;
 					return -1;
@@ -165,7 +165,7 @@ static int php_stream_memory_seek(php_stream *stream, zend_off_t offset, int whe
 				stream->eof = 0;
 				stream->fatal_error = 0;
 				return 0;
-			} else if (ZSTR_LEN(ms->data) < (size_t)(-offset)) {
+			} else if (ZSTR_LEN(ms->data) < -(size_t)offset) {
 				ms->fpos = 0;
 				*newoffs = -1;
 				return -1;

tests/basic/bug20964.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Bug #20964 fseek with PHP_INT_MIN on php://memory
+--FILE--
+<?php
+$stream = fopen('php://memory', 'r+');
+
+$result = fseek($stream, PHP_INT_MIN, SEEK_END);
+var_dump($result);
+?>
+--EXPECTF--
+int(%d)