avcodec/wmalosslessdec: Reset num_saved_bits on error path

Fixes: NULL pointer dereference
Fixes: poc-201803.wav
Found-by: GwanYeong Kim <gy741.kim@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 64c9ce0abc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wmalosslessdec.c

@@ -1148,6 +1148,7 @@ static void save_bits(WmallDecodeCtx *s, GetBitContext* gb, int len,
     if (len <= 0 || buflen > s->max_frame_size) {
         avpriv_request_sample(s->avctx, "Too small input buffer");
         s->packet_loss = 1;
+        s->num_saved_bits = 0;
         return;
     }