Avoid unnecessary failures of auto-saving after fatal error

Avoid unnecessary failures of auto-saving after fatal error

* src/w32.c (map_w32_filename): Avoid non-trivial system calls for
the benefit of FAT volumes if we are called as part of shutting
down due to a fatal error, which probably means we are trying to
auto-save the session.
* src/lread.c (check_obarray): Don't bother making the obarray
valid if we are shutting down due to a fatal error.  This avoids
interfering with auto-saving the crashed session.