clang compiler warnings: Fix sometimes-initialized warning in func_math

This patch fixes a bug in a unit test in func_math where a variable could be
passed to ast_free that wasn't allocated. This patch corrects the issue and
ensures that we only attempt to free a variable if we previously allocated
it.

Review: https://reviewboard.asterisk.org/r/4552

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4552.patch submitted by dkdegroot (License 6600)
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Merged revisions 434190 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 434191 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434192 65c4cc65-6c06-0410-ace0-fbb531ad65f3