res/res_phoneprov: Bail on registration if res_phoneprov didn't load

If res_phoneprov failed to fully load (due to not being configured), the
providers container will be NULL. If a module attempts to register a phone
provisioning provider, it should check for the presence of the container.
If there is no providers container, it should return an error.

This patch makes the ast_phoneprov_provider_register function do that...
otherwise this would be a silly commit message.
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Merged revisions 425220 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 425221 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@425222 65c4cc65-6c06-0410-ace0-fbb531ad65f3