Fix bad check in voicemail functions for ast_inboxcount2_func

Check looks for ast_inboxcount_func instead of ast_inboxcount2_func on
ast_inboxcount2_func calls.

(closes issue ASTERISK-19718)
Reported by: Corey Farrell
Patches:
	ast_app_inboxcount2-null-refcheck.patch uploaded by Corey Farrell (license 5909)
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Merged revisions 364769 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 364777 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@364785 65c4cc65-6c06-0410-ace0-fbb531ad65f3