rv34: Avoid NULL dereference on corrupted bitstream

rv34_decode_slice() can return without allocating any pictures.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit d0f6ab0298)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/rv34.c

@@ -1486,7 +1486,7 @@ int ff_rv34_decode_frame(AVCodecContext *avctx,
             break;
     }
 
-    if(last){
+    if(last && s->current_picture_ptr){
         if(r->loop_filter)
             r->loop_filter(r, s->mb_height - 1);
         ff_er_frame_end(s);