keyfile: return 0 when the parsed double value is invalid

As specified in the g_key_file_get_double documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=768504

glib/gkeyfile.c

@@ -4345,6 +4345,8 @@ g_key_file_parse_value_as_double  (GKeyFile     *key_file,
 		     "as a float number."), 
 		   value_utf8);
       g_free (value_utf8);
+
+      double_value = 0;
     }
 
   return double_value;