Avoid some core dumps in X session management

Avoid some core dumps in X session management

Derived from a bug report by Nicolas Richard in:
http://bugs.gnu.org/20191#20
* xsmfns.c (smc_save_yourself_CB): Don't dump core if
invocation-name is not a string.  Initialize user-login-name if it
is not already initialized, and don't dump core if it is not a
string.
(create_client_leader_window): Don't dump core if x-resource-name
and x-resource-class are not both strings.
(x_session_initialize): Don't dump core if x-session-previous-id,
invocation-directory, and invocation-name are not strings.