avcodec/mpeg12dec: Do not call show_bits() with invalid bits

Fixes assertion failure
Fixes: 63e50545709a6440d3d59f6426d58db9/signal_sigabrt_7ffff6ae7cc9_8189_3272a3010fd98ddf947c662bbde1ac13.ts

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 973c3dba)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mpeg12dec.c

@@ -1909,7 +1909,7 @@ static int mpeg_decode_slice(MpegEncContext *s, int mb_y,
                     (left && show_bits(&s->gb, FFMIN(left, 23)) && !is_d10) ||
                     ((avctx->err_recognition & (AV_EF_BITSTREAM | AV_EF_AGGRESSIVE)) && left > 8)) {
                     av_log(avctx, AV_LOG_ERROR, "end mismatch left=%d %0X\n",
-                           left, show_bits(&s->gb, FFMIN(left, 23)));
+                           left, left>0 ? show_bits(&s->gb, FFMIN(left, 23)) : 0);
                     return -1;
                 } else
                     goto eos;