avcodec/ralf: Fix undefined pointer in decode_channel()

Fixes: 16203/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RALF_fuzzer-5086088934195200

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3c06ba1716)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ralf.c

@@ -286,7 +286,7 @@ static int decode_channel(RALFContext *ctx, GetBitContext *gb, int ch,
             add_bits--;
         range    = 10;
         range2   = 21;
-        code_vlc = set->long_codes + code_params - 15;
+        code_vlc = set->long_codes + (code_params - 15);
     } else {
         add_bits = 0;
         range    = 6;