ffmpeg: Fix cleanup with ost = NULL

Fixes: 09e670595acbdafb226974b08dab66e3_signal_sigabrt_7ffff70eccc9_991_xtrem_e2_m64q15_a32sxx.3gp with memlimit of 1048576

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

ffmpeg.c

@@ -510,7 +510,12 @@ static void ffmpeg_cleanup(int ret)
     }
     for (i = 0; i < nb_output_streams; i++) {
         OutputStream *ost = output_streams[i];
-        AVBitStreamFilterContext *bsfc = ost->bitstream_filters;
+        AVBitStreamFilterContext *bsfc;
+
+        if (!ost)
+            continue;
+
+        bsfc = ost->bitstream_filters;
         while (bsfc) {
             AVBitStreamFilterContext *next = bsfc->next;
             av_bitstream_filter_close(bsfc);