lavd video: don't print false positive errs

do not print errors like:

    uv -d dummy -t testcard -c libavcodec
    [lavd] Error while decoding frame (rc == 42836): Error number 42836 occurred.

when decoder returns empty packet ("error" code acutally indicated number
of consumed bytes).

src/video_decompress/libavcodec.c

@@ -996,6 +996,7 @@ decode_frame(struct state_libavcodec_decompress *s, unsigned char *src,
                         }
                 }
                 if (s->pkt->size == 0) {
+                        ret = 0;
                         break;
                 }
                 src += ret;