RADIUS server: Fix error paths in new session creation

RADIUS server: Fix error paths in new session creation

radius_server_session_free() does not remove the session from the
session list and these radius_server_get_new_session() error paths ended
up leaving a pointer to freed memory into the session list. This
resulted in the following operations failing due to use of freed memory.

Fix this by using radius_server_session_remove() which removes the entry
from the list in addition to calling radius_server_session_free().

Signed-off-by: Jouni Malinen <j@w1.fi>