Fix RT #97625 use-after-free in mysql_dr_error, and #86153

free imp_dbh->pmysql  in my_login matching the allocation.
In the success case, imp_dbh->pmysql is freed seperately
later in dbd_db_destroy().
This reverts the wrong leak-fix from RT #86153 and adds the
real fix for it.
Merge pull request #26 from rurban/rt-97625

Fix RT #97625 use-after-free in mysql_dr_error, and #86153