avcodec/mpegvideo: Check for NULL in ff_mpv_common_end()

Fixes: af94b3a3d26586c08f557cafe8246251_signal_sigsegv_7ffff713351a_343_XFMode.ASF with 1048576 alloc limit

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

libavcodec/mpegvideo.c

@@ -1107,6 +1107,9 @@ void ff_mpv_common_end(MpegEncContext *s)
 {
     int i;
 
+    if (!s)
+        return ;
+
     if (s->slice_context_count > 1) {
         for (i = 0; i < s->slice_context_count; i++) {
             free_duplicate_context(s->thread_context[i]);