wmaprodec: Fix null pointer dereference in decode_frame()

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wmaprodec.c

@@ -1581,7 +1581,8 @@ static int decode_packet(AVCodecContext *avctx, void *data,
             (frame_size = show_bits(gb, s->log2_frame_size)) &&
             frame_size <= remaining_bits(s, gb)) {
             save_bits(s, gb, frame_size, 0);
-            s->packet_done = !decode_frame(s, data, got_frame_ptr);
+            if (!s->packet_loss)
+                s->packet_done = !decode_frame(s, data, got_frame_ptr);
         } else if (!s->len_prefix
                    && s->num_saved_bits > get_bits_count(&s->gb)) {
             /** when the frames do not have a length prefix, we don't know