owornmask sanity_check fix and enhancement

owornmask sanity_check fix and enhancement

Thinko fix:  sanity checking for owornmask was mis-treating
OBJ_MIGRATING as OBJ_MINVENT of migrating monsters rather than as
unattended objects and would have had problems similar to obfree's
inappropriate impossible check.

Sanity checking for objects worn in invalid slots (amulet worn in
a ring slot and so forth) is extended to items worn by monsters.
Also add a check for wielded coins since the loophole that let them
become wielded has been closed.