avcodec/interplayacm: Fix overflow of last unused value

Fixes: signed integer overflow: -2147450880 - 65535 cannot be represented in type 'int'
Fixes: 18393/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_INTERPLAY_ACM_fuzzer-5667520110919680

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

libavcodec/interplayacm.c

@@ -529,7 +529,7 @@ static int decode_block(InterplayACMContext *s)
 
     for (i = 1, x = -val; i <= count; i++) {
         s->midbuf[-i] = x;
-        x -= val;
+        x -= (unsigned)val;
     }
 
     ret = fill_block(s);