ipsecmod: fix deref on null in ipsecmod-whitelist after OOM (#1475)

DEREF_OF_NULL.RET.STAT Return value of a function 'rbtree_create'
is dereferenced at ipsecmod-whitelist.c:105 without checking for
NULL, but it is usually checked for this function (5/6).

In ipsecmod_whitelist_apply_cfg(), the return value of rbtree_create()
is not checked for NULL before being used.

Found by the static analyzer Svace (ISP	RAS).

Signed-off-by: Petr Vaganov <petrvaganoff@gmail.com>