config: Fix inverted test

The test of the result of the stat() call was inverted such that its
output was only used if the call failed. This inverts the test so that
the output of stat() is used correctly. This was causing full reloads
on unchanged files.

(closes issue ASTERISK-23383)
Reported by: David Woolley


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409916 f38db490-d61c-443f-a65b-d21fe96a405b