avcodec/huffyuvdec: avoid undefined behavior with get_vlc2() failure

Fixes: left shift of negative value -1
Fixes: 59889/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HUFFYUV_fuzzer-5472742275940352

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/huffyuvdec.c

@@ -757,7 +757,7 @@ static void decode_plane_bitstream(HYuvDecContext *s, int width, int plane)
             }
         }
         if( width&1 && get_bits_left(&s->gb)>0 ) {
-            int dst = get_vlc2(&s->gb, s->vlc[plane].table, VLC_BITS, 3)<<2;
+            int dst = (unsigned)get_vlc2(&s->gb, s->vlc[plane].table, VLC_BITS, 3)<<2;
             s->temp16[0][width-1] = dst + get_bits(&s->gb, 2);
         }
     }