clang compiler warnings: Fix sometimes-uninitialized warning in pbx_config

This patch fixes a warning caught by clang, in which a char pointer could be
assigned to before it was initialized. The patch re-organizes the code to
ensure that the pointer is always initialized, even on off nominal paths.

Review: https://reviewboard.asterisk.org/r/4529

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4529.patch submitted by dkdegroot (License 6600)
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Merged revisions 434090 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 434091 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434092 65c4cc65-6c06-0410-ace0-fbb531ad65f3