avcodec/rka: Fix signed integer overflow in decode_filter()

Fixes: signed integer overflow: -631553 * 32768 cannot be represented in type 'int'
Fixes: 57814/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RKA_fuzzer-4614661233573888

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

libavcodec/rka.c

@@ -737,7 +737,7 @@ static int decode_filter(RKAContext *s, ChContext *ctx, ACoder *ac, int off, uns
                 }
                 ctx->buf0[off] = ctx->buf1[off] + ctx->buf0[off + -1];
             } else {
-                val *= 1 << ctx->cmode;
+                val *= 1U << ctx->cmode;
                 sum += ctx->buf0[off + -1] + val;
                 switch (s->bps) {
                 case 16: sum = av_clip_int16(sum); break;