avcodec/vp3: check current_frame before accessing it

Fixes null pointer dereference
Fixes: b15eb06e0111e94bc59123c86db7aff9_signal_sigsegv_a0500f_45_320vp3.nsv with allocation limit  536870912

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

libavcodec/vp3.c

@@ -1947,6 +1947,8 @@ static int vp3_update_thread_context(AVCodecContext *dst, const AVCodecContext *
     }
 
     if (s != s1) {
+        if (!s->current_frame.f)
+            return AVERROR(ENOMEM);
         // init tables if the first frame hasn't been decoded
         if (!s->current_frame.f->data[0]) {
             int y_fragment_count, c_fragment_count;