ffmpeg: Fix crash with ost->last_frame allocation failure

Fixes: 1013dbde2c360d939cc2dfc33e4f275c_signal_sigsegv_a0500f_45_320vp3.nsv with memlimit of 536870912

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

ffmpeg.c

@@ -1227,7 +1227,7 @@ static void do_video_out(AVFormatContext *s,
     if (!ost->last_frame)
         ost->last_frame = av_frame_alloc();
     av_frame_unref(ost->last_frame);
-    if (next_picture)
+    if (next_picture && ost->last_frame)
         av_frame_ref(ost->last_frame, next_picture);
     else
         av_frame_free(&ost->last_frame);