avcodec/dxtory: Fix negative stride shift in dx2_decode_slice_420()

Fixes: left shift of negative value -640
Fixes: 26044/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DXTORY_fuzzer-5631057602543616

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

libavcodec/dxtory.c

@@ -717,7 +717,7 @@ static int dx2_decode_slice_420(GetBitContext *gb, AVFrame *frame,
             V[huvborder] = decode_sym(gb, lru[2]) ^ 0x80;
         }
 
-        Y += ystride << 1;
+        Y += ystride * 2;
         U += ustride;
         V += vstride;
     }