avcodec/wmalosslessdec: Fix loop in revert_acfilter()

Fixes: out of array read
Fixes: 20059/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-5691776237305856

No testcase except the fuzzed one.

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

libavcodec/wmalosslessdec.c

@@ -823,7 +823,10 @@ static void revert_acfilter(WmallDecodeCtx *s, int tile_size)
             pred >>= scaling;
             s->channel_residues[ich][i] += (unsigned)pred;
         }
-        for (j = 0; j < order; j++)
+        for (j = order - 1; j >= 0; j--)
+            if (tile_size <= j) {
+                prevvalues[j] = prevvalues[j - tile_size];
+            }else
                 prevvalues[j] = s->channel_residues[ich][tile_size - j - 1];
     }
 }