Ensure each dive has its own copy of the location text (UEMIS importer)

Since multiple dives can reference the same divesite we need to do the
strdup when the value gets assigned, not when it gets passed into the
helper function.

This also validates the location string as on my divecomputer there is an
invalid divespot 0 that has a corrupted UTF8 string as location name.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>