fftools/ffmpeg: Fix crash when flushing non-fully setup output stream

The output stream's packet may not have been allocated
at that point. This happens when quitting in the following command line:
$ ./ffmpeg -lavfi abuffer=sample_fmt=u8:sample_rate=48000:channel_layout=stereo -f null -

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit fb215798c7)

fftools/ffmpeg.c

@@ -1974,6 +1974,9 @@ static void flush_encoders(void)
             AVPacket *pkt = ost->pkt;
             int pkt_size;
 
+            if (!pkt)
+                break;
+
             switch (enc->codec_type) {
             case AVMEDIA_TYPE_AUDIO:
                 desc   = "audio";