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

We try to keep the system running even when all available memory is
spent.

Review: https://reviewboard.asterisk.org/r/2734/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@396279 f38db490-d61c-443f-a65b-d21fe96a405b