avcodec/ralf: use multiply instead of shift to avoid undefined behavior in decode_block()

Fixes: left shift of negative value -249
Fixes: 18566/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RALF_fuzzer-5649394561187840

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

libavcodec/ralf.c

@@ -408,7 +408,7 @@ static int decode_block(AVCodecContext *avctx, GetBitContext *gb,
     case 4:
         for (i = 0; i < len; i++) {
             t  =   ch1[i] + ctx->bias[1];
-            t2 = ((ch0[i] + ctx->bias[0]) << 1) | (t & 1);
+            t2 = ((ch0[i] + ctx->bias[0]) * 2) | (t & 1);
             dst0[i] = (t2 + t) / 2;
             dst1[i] = (t2 - t) / 2;
         }