avcodec/rka: avoid undefined multiply in cmode==0

Fixes: signed integer overflow: -182838 * 32768 cannot be represented in type 'int'
Fixes: 58179/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RKA_fuzzer-5333265899978752

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

@@ -732,7 +732,7 @@ static int decode_filter(RKAContext *s, ChContext *ctx, ACoder *ac, int off, uns
                 if (bits == 0) {
                     ctx->buf1[off] = sum + val;
                 } else {
-                    ctx->buf1[off] = (val + (sum >> bits)) * (1 << bits) +
+                    ctx->buf1[off] = (val + (sum >> bits)) * (1U << bits) +
                         (((1U << bits) - 1U) & ctx->buf1[off + -1]);
                 }
                 ctx->buf0[off] = ctx->buf1[off] + ctx->buf0[off + -1];