fix: Use $memID as applicator if coming from registration
fix: Use $memID as applicator if coming from registration

 `$user_info` is populated with guest credentials when members register, so cannot be used to set custom profile fields.

Fixes #6237
Closes #6486

StealthWombat/SMF2.1-more-forkery,StealthWombat/SMF2.1-more-forkery