NR: Refactor the White Magi respawn location finding code into a macro

This bit was subject to a particularly bad case of code copy-pasting
when the only real variable is the id of the first respawn hub
candidate.

Added a statement to clear the respawn_hub WML variable after running
the code since it's never used again afterwards.
NR: Search for White Magi respawn locs only within map edges (bug #22097)

Without this, the engine may match map borders and send the respawnee
back to the player's recall list as those count as invalid locations
like "recall","recall".