Fix deadlock when selecting encrypt

* src/mail.cpp (Mail::locateAllCryptoRecipients_o): Use a copy
of the mail map instead of a locked instance.

Locking the mail map for a complex operation is a bad
idea. What happened here is that locateKeys_o can start
different threads and those threads can use "Mail::isValidPtr" which
uses the s_mail_map and locks it accordingly.
So there is a deadlock.