idroqdec: fix leaking pkt on failure

The code calls av_new_packet a few lines above and the allocated memory
has to be freed in case of an error.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/idroqdec.c

@@ -222,8 +222,10 @@ static int roq_read_packet(AVFormatContext *s,
             pkt->pos= avio_tell(pb);
             ret = avio_read(pb, pkt->data + RoQ_CHUNK_PREAMBLE_SIZE,
                 chunk_size);
-            if (ret != chunk_size)
+            if (ret != chunk_size) {
+                av_packet_unref(pkt);
                 ret = AVERROR(EIO);
+            }
 
             packet_read = 1;
             break;