Fix OOB gzseek() causing assertion failure

Author: ndossche

ext/zlib/tests/gzseek_seek_oob.phpt

@@ -13,8 +13,7 @@ var_dump(gztell($h));
 
 gzclose($h);
 ?>
---EXPECTF--
+--EXPECT--
 int(-1)
-php: %s: _php_stream_seek: Assertion `stream->position >= 0' failed.
-
-Termsig=6
+int(0)
+int(0)

ext/zlib/zlib_fopen_wrapper.c

@@ -94,9 +94,14 @@ static int php_gziop_seek(php_stream *stream, zend_off_t offset, int whence, zen
 		php_error_docref(NULL, E_WARNING, "SEEK_END is not supported");
 		return -1;
 	}
-	*newoffs = gzseek(self->gz_file, offset, whence);
 
-	return (*newoffs < 0) ? -1 : 0;
+	off_t new_offset = gzseek(self->gz_file, offset, whence);
+	if (new_offset < 0) {
+		return -1;
+	}
+
+	*newoffs = new_offset;
+	return 0;
 }
 
 static int php_gziop_close(php_stream *stream, int close_handle)