res_pjsip_phoneprov_provider: Fix reference leak on unload

res_pjsip_phoneprov_provider was leaking references to phoneprov objects due to 
a missing OBJ_NODATA in an ao2_callback in load_users().  Rather than adding the 
OBJ_NODATA, I changed load_users to use a more straightforward ao2_iterator.  
This plugged the leak but exposed an unload order issue between 
res_pjsip_phoneprov_provider, res_phoneprov and res_pjsip.

res_pjsip_phoneprov_provider unloads first, then res_phoneprov, then res_pjsip.  
Since res_pjsip_phoneprov_provider uses res_pjsip's sorcery instance, when it 
unloads, it's objects are still in the sorcery instance.  When res_pjsip 
unloads, it destroys all its objects including res_pjsip_phoneprov_provider's.  
The phoneprov destructor then attempts to unregister the extension from 
res_phoneprov but because res_phoneprov is already cleaned up, its users 
container is gone and we get a FRACK.

Simple solution, check for the NULL users container before attempting to remove 
the entry. Duh.

Ran tests/res_phoneprov/res_phoneprov_provider.  No leaks in 
res_pjsip_phoneprov_provider and no FRACKs.

Reported-by: Corey Farrell
Tested-by: George Joseph
Review: https://reviewboard.asterisk.org/r/4608/
ASTERISK-24935 #close



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434545 65c4cc65-6c06-0410-ace0-fbb531ad65f3