gxfenc: fix null ptr dereference

Fixes CID703677
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/gxfenc.c

@@ -796,7 +796,7 @@ static int gxf_write_header(AVFormatContext *s)
     if (ff_audio_interleave_init(s, GXF_samples_per_frame, (AVRational){ 1, 48000 }) < 0)
         return -1;
 
-    if (tcr)
+    if (tcr && vsc)
         gxf_init_timecode(s, &gxf->tc, tcr->value, vsc->fields);
 
     gxf_init_timecode_track(&gxf->timecode_track, vsc);