Correct the return value check for ast_safe_system.

The logic here was reversed as ast_safe_system returns -1 on error and not on
success.  Fix suggested by reporter.

(closes issue #15667)
Reported by: loic


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@212574 65c4cc65-6c06-0410-ace0-fbb531ad65f3