avcodec/tta: Fix undefined shift

Fixes: left shift of negative value -4483
Fixes: 15256/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TTA_fuzzer-5738691617619968

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

libavcodec/tta.c

@@ -372,7 +372,7 @@ static int tta_decode_frame(AVCodecContext *avctx, void *data,
         // shift samples for 24-bit sample format
         int32_t *samples = (int32_t *)frame->data[0];
         for (i = 0; i < framelen * s->channels; i++)
-            *samples++ <<= 8;
+            *samples++ *= 256;
         // reset decode buffer
         s->decode_buffer = NULL;
         break;