gunixmounts: Fix crash if path doesn't exist
gunixmounts: Fix crash if path doesn't exist

g_unix_mount_for segfaults for nonexistent paths. Return NULL if path
doesn't exist.

https://bugzilla.gnome.org/show_bug.cgi?id=772160

johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib