Fix file descriptor check so the check works as required even if
file descriptor 0 is used for the file that is being read.

diff -ur a/glib/gkeyfile.c b/glib/gkeyfile.c
--- a/glib/gkeyfile.c	2015-11-24 03:13:44.000000000 +0000
+++ b/glib/gkeyfile.c	2016-01-08 14:25:52.609264298 +0000
@@ -738,7 +738,7 @@
                              "found in search dirs"));
     }
 
-  if (output_file != NULL && fd > 0)
+  if (output_file != NULL && fd != -1)
     *output_file = g_strdup (path);
 
   g_free (path);
