avutil/frame: fix crash with av_frame_unref(NULL)

Fixes: af94b3a3d26586c08f557cafe8246251_signal_sigsegv_7ffff713351a_343_XFMode.ASF with 2097152 alloc limit

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

libavutil/frame.c

@@ -463,6 +463,9 @@ void av_frame_unref(AVFrame *frame)
 {
     int i;
 
+    if (!frame)
+        return;
+
     wipe_side_data(frame);
 
     for (i = 0; i < FF_ARRAY_ELEMS(frame->buf); i++)