avcodec/exr: skip bottom clearing loop when its outside the image

Fixes: signed integer overflow: 1633771809 * 32960 cannot be represented in type 'int'
Fixes: 26532/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-5613925708857344

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