avcodec/iff: Check available space before entering loop in decode_long_vertical_delta2() / decode_long_vertical_delta()

Fixes: Timeout (31sec -> 41ms)
Fixes: 18380/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_IFF_ILBM_fuzzer-5645210121404416

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