avcodec/iff: Fix several integer overflows

Fixes: negation of -2147483648 cannot be represented in type 'int32_t' (aka 'int'); cast to an unsigned type to negate this value to itself
Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int32_t' (aka 'int')
Fixes: 20492/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5764066459254784

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

libavcodec/iff.c

@@ -1378,11 +1378,10 @@ static void decode_delta_d(uint8_t *dst,
                     opcode--;
                 }
             } else {
-                opcode = -opcode;
                 while (opcode && bytestream2_get_bytes_left(&gb) > 0) {
                     bytestream2_put_be32(&pb, bytestream2_get_be32(&gb));
                     bytestream2_skip_p(&pb, pitch - 4);
-                    opcode--;
+                    opcode++;
                 }
             }
             entries--;