Add safety NULL pointer check in module user references.

Add safety NULL pointer check in module user references.

Made __ast_module_user_remove() check for NULL pointers.
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Merged revision 375860 from C.3
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Merged revisions 375862 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375863 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375864 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375865 65c4cc65-6c06-0410-ace0-fbb531ad65f3