avcodec/exr: Check tile positions

This also disabled the case of mixed x/ymin with tiles, the code
handles these cases inconsistent for the 2 coordinate axis and is
unlikely working correctly.

Fixes crash
Fixes: poc1.exr, poc2.exr

Found-by: Yaoguang Chen of Aliapy unLimit Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
