Fix memleaks

* rlm_mongodb.c (mongodb_getvpdata):
  - Fix memleak, the bson cursor should be freed
* rlm_mongodb.c (mongodb_process_groups):
  - Move pairfree() outside the pair compare success as it
    should free the temporary valure-pair even compare fail.

Signed-off-by: Neutron Soutmun <neo.neutron@gmail.com>