vaapidecode: sets return value in failure case.

In gst_vaapidecode_handle_frame, when there is a decode error
there is a code path the returns an uninitialized value.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=797006

gst/vaapi/gstvaapidecode.c

@@ -760,6 +760,7 @@ error_decode:
         ret = GST_FLOW_NOT_SUPPORTED;
         break;
       default:
+        ret = GST_FLOW_OK;
         GST_VIDEO_DECODER_ERROR (vdec, 1, STREAM, DECODE, ("Decoding error"),
             ("Decode error %d", status), ret);
         break;