Check result of ast_var_assign() calls for memory allocation failure (2).

Missed a spot in the previous commit.
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Merged revisions 396310 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396311 65c4cc65-6c06-0410-ace0-fbb531ad65f3