BUG/MINOR: check_config_validity: check the returned value of stktable_init()

The function stktable_init() will return 0 if create_pool() returns NULL. Since
the returned value of this function is ignored, HAProxy will crash if the pool
of stick table is NULL and stksess_new() is called to allocate a new stick
session. It is a better choice to check the returned value and make HAProxy exit
with alert message if any error is caught.

Signed-off-by: Godbach <nylzhaowei@gmail.com>