pbx.c: Pre-initialize timezone to avoid crash on destroy

In ast_build_timing, initialize the timezone value to NULL
in order to avoid deferencing an uninitialized value later
when calling ast_destroy_timing.  The timezone value could
be uninitialized if ast_build_timing were to fail due to a
zero length time string.

(closes issue ASTERISK-22861)
Reported by: Sebastian Murray-Roberts
Review: https://reviewboard.asterisk.org/r/3134/
Patches:
     ast_build_timing-initialize-timezone.patch uploaded by coreyfarrell (license 5909)
........

Merged revisions 406241 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@406245 f38db490-d61c-443f-a65b-d21fe96a405b