avcodec/rka: avoid undefined doubling sum overflow

Fixes: signed integer overflow: -2124073172 * 2 cannot be represented in type 'int'
Fixes: 56099/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RKA_fuzzer-4530933127839744

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

@@ -724,7 +724,7 @@ static int decode_filter(RKAContext *s, ChContext *ctx, ACoder *ac, int off, uns
             src = &ctx->buf1[off + -1];
             for (int i = 0; i < filt.size && i < 15; i++)
                 sum += filt.coeffs[i] * (unsigned)src[-i];
-            sum = sum * 2;
+            sum = sum * 2U;
             for (int i = 15; i < filt.size; i++)
                 sum += filt.coeffs[i] * (unsigned)src[-i];
             sum = sum >> 6;