avcodec/iff: Check length before memcpy() in decode_deep_rle32()

Fixes: out of array read
Fixes: 20796/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5111364702175232.fuzz

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

@@ -722,6 +722,8 @@ static void decode_deep_rle32(uint8_t *dst, const uint8_t *src, int src_size, in
             int size = opcode + 1;
             for (i = 0; i < size; i++) {
                 int length = FFMIN(size - i, width);
+                if (src_end - src < length * 4)
+                    return;
                 memcpy(dst + y*linesize + x * 4, src, length * 4);
                 src += length * 4;
                 x += length;