Malformed chm file will trigger a floating point exception / crash

[hannob]

This file will trigger a floating point exception in chmlib:
https://crashes.fuzzing-project.org/chmlib-floating-point-exception.chm

Can be tested with enum_chmLib [file]. This was found with the tool american fuzzy lop.

[jedwing]

Thanks for the report! Looks like there are a few different places that could trigger this, but as a class of errors, it seems straightforward to fix.