avcodec/ralf: Fix overflows of biased values

Fixes: signed integer overflow: 2003010644 * 2 cannot be represented in type 'int'
Fixes: 19593/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RALF_fuzzer-5660628006207488

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

@@ -60,7 +60,7 @@ typedef struct RALFContext {
     int     filter_bits;     ///< filter precision for the current channel data
     int32_t filter[64];
 
-    int     bias[2];         ///< a constant value added to channel data after filtering
+    unsigned bias[2];        ///< a constant value added to channel data after filtering
 
     int num_blocks;          ///< number of blocks inside the frame
     int sample_offset;