[PPC] Add missing SLB allocation KASSERT

Although PPC SLB code doesn't handle allocation failures,
which are rare, in most places it asserts that the pointer
returned by uma_zalloc() is not NULL, making it easier to
identify the failure and avoiding an invalid pointer dereference.

This change simply adds a missing KASSERT in SLB code.

sys/powerpc/aim/slb.c

@@ -416,6 +416,7 @@ slb_alloc_tree(void)
 	struct slbtnode *root;
 
 	root = uma_zalloc(slbt_zone, M_NOWAIT | M_ZERO);
+	KASSERT(root != NULL, ("unhandled NULL case"));
 	root->ua_level = UAD_ROOT_LEVEL;
 
 	return (root);