matroska: do not return GST_FLOW_OK if we did not get a buffer

Coverity 1139714 (which will likely come back in another guise,
as the _read_init call can have a failing _map)

gst/matroska/matroska-parse.c

@@ -2282,6 +2282,8 @@ gst_matroska_parse_take (GstMatroskaParse * parse, guint64 bytes,
     gst_ebml_read_init (ebml, GST_ELEMENT_CAST (parse), buffer,
         parse->common.offset);
     parse->common.offset += bytes;
+  } else {
+    ret = GST_FLOW_ERROR;
   }
 exit:
   return ret;