avformat/test/fifo_muxer: add check for FailingMuxerPacketData alloc

CID: 1396257
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
(cherry picked from commit d1f3e475)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/tests/fifo_muxer.c

@@ -150,6 +150,9 @@ static int prepare_packet(AVPacket *pkt,const FailingMuxerPacketData *pkt_data,
 {
     int ret;
     FailingMuxerPacketData *data = av_malloc(sizeof(*data));
+    if (!data) {
+        return AVERROR(ENOMEM);
+    }
     memcpy(data, pkt_data, sizeof(FailingMuxerPacketData));
     ret = av_packet_from_data(pkt, (uint8_t*) data, sizeof(*data));