pbx.c: Make pbx_substitute_variables_helper_full() not mask variables.

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Merged revisions 397977 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397978 65c4cc65-6c06-0410-ace0-fbb531ad65f3