rtpdec_h263_rfc2190: Clear the stored bits if discarding buffered data

If we throw away the buffered incomplete frame, make sure to also
throw away the buffered bits of an incomplete byte at the same
time.

(cherry picked from commit df07c07b)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/rtpdec_h263_rfc2190.c

@@ -83,6 +83,7 @@ static int h263_handle_packet(AVFormatContext *ctx, PayloadContext *data,
         avio_close_dyn_buf(data->buf, &p);
         av_free(p);
         data->buf = NULL;
+        data->endbyte_bits = 0;
     }
 
     if (len < 4) {