selinux: fix NULL-pointer dereference when hashtab allocation fails

In the Linux kernel, the following vulnerability has been resolved:

selinux: fix NULL-pointer dereference when hashtab allocation fails

When the hash table slot array allocation fails in hashtab_init(),

h->size is left initialized with a non-zero value, but the h->htable

pointer is NULL. This may then cause a NULL pointer dereference, since

the policydb code relies on the assumption that even after a failed

hashtab_init(), hashtab_map() and hashtab_destroy() can be safely called

on it. Yet, these detect an empty hashtab only by looking at the size.

Fix this by making sure that hashtab_init() always leaves behind a valid

empty hashtab when the allocation fails.