Fix GH-19705: do not flush/write buffer on non writeable stream.

Co-authored-by: Jakub Zelenka <bukka@php.net>

close GH-19708

NEWS

@@ -57,6 +57,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-19248 (Use strerror_r instead of strerror in main).
     (Jakub Zelenka)
   . Fixed bug GH-17345 (Bug #35916 was not completely fixed). (nielsdos)
+  . Fixed bug GH-19705 (segmentation when attempting to flush on non seekable
+    stream. (bukka/David Carlier)
 
 - XMLReader:
   . Fixed bug GH-20009 (XMLReader leak on RelaxNG schema failure). (nielsdos)

ext/standard/tests/streams/gh19705.phpt

@@ -0,0 +1,11 @@
+--TEST--
+GH-19705 segmentation fault with non writable stream at stream_filter_append call.
+--EXTENSIONS--
+zlib
+--FILE--
+<?php
+$fh = fopen('data://text/plain,', 'w+',);
+var_dump(stream_filter_append($fh, 'zlib.deflate',STREAM_FILTER_WRITE));
+?>
+--EXPECTF--
+resource(%d) of type (stream filter)

main/streams/streams.c

@@ -1295,7 +1295,7 @@ PHPAPI int _php_stream_flush(php_stream *stream, int closing)
 {
 	int ret = 0;
 
-	if (stream->writefilters.head) {
+	if (stream->writefilters.head && stream->ops->write) {
 		_php_stream_write_filtered(stream, NULL, 0, closing ? PSFS_FLAG_FLUSH_CLOSE : PSFS_FLAG_FLUSH_INC );
 	}