Fix phar crash and file corruption with SplFileObject

There are two bugfixes here.
The first was a crash that I discovered while working on phpGH-19035.
The check for when a file pointer was still occupied was wrong, leading
to a UAF. Strangely, zip got this right.

The second issue was that even after fixing the first one, the file
contents were garbage. This is because the file write offset for the
phar stream was wrong.

Author: nielsdos

ext/phar/phar.c

@@ -2737,7 +2737,7 @@ int phar_flush(phar_archive_data *phar, char *user_stub, zend_long len, int conv
 			/* remove this from the new phar */
 			continue;
 		}
-		if (!entry->is_modified && entry->fp_refcount) {
+		if (entry->fp_refcount) {
 			/* open file pointers refer to this fp, do not free the stream */
 			switch (entry->fp_type) {
 				case PHAR_FP:

ext/phar/stream.c

@@ -450,12 +450,12 @@ static ssize_t phar_stream_write(php_stream *stream, const char *buf, size_t cou
 {
 	phar_entry_data *data = (phar_entry_data *) stream->abstract;
 
-	php_stream_seek(data->fp, data->position, SEEK_SET);
+	php_stream_seek(data->fp, data->position + data->zero, SEEK_SET);
 	if (count != php_stream_write(data->fp, buf, count)) {
 		php_stream_wrapper_log_error(stream->wrapper, stream->flags, "phar error: Could not write %d characters to \"%s\" in phar \"%s\"", (int) count, data->internal_file->filename, data->phar->fname);
 		return -1;
 	}
-	data->position = php_stream_tell(data->fp);
+	data->position = php_stream_tell(data->fp) - data->zero;
 	if (data->position > (zend_off_t)data->internal_file->uncompressed_filesize) {
 		data->internal_file->uncompressed_filesize = data->position;
 	}

ext/phar/tar.c

@@ -832,7 +832,7 @@ static int phar_tar_writeheaders_int(phar_entry_info *entry, void *argument) /*
 		php_stream_write(fp->new, padding, ((entry->uncompressed_filesize +511)&~511) - entry->uncompressed_filesize);
 	}
 
-	if (!entry->is_modified && entry->fp_refcount) {
+	if (entry->fp_refcount) {
 		/* open file pointers refer to this fp, do not free the stream */
 		switch (entry->fp_type) {
 			case PHAR_FP:

ext/phar/tests/gh19038.phpt

@@ -0,0 +1,25 @@
+--TEST--
+GH-19038 (Phar crash and data corruption with SplFileObject)
+--EXTENSIONS--
+phar
+--INI--
+phar.readonly=0
+--FILE--
+<?php
+
+$phar = new Phar(__DIR__ . "/gh19038.phar");
+$phar->addFromString("file", "123");
+$file = $phar["file"]->openFile();
+$file->fseek(3);
+var_dump($file->fwrite("456", 3));
+$file->fseek(0);
+echo $file->fread(100);
+
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . "/gh19038.phar");
+?>
+--EXPECT--
+int(3)
+123456