mem-pool: Reject the creation of unintentionally empty pools

If a base address is configured, we don't expect the pool to be empty,
so reject the creation (e.g. with the broadcast address as base).

References strongswan/strongswan#2205

src/libcharon/tests/suites/test_mem_pool.c

@@ -153,13 +153,10 @@ START_TEST(test_cidr_offset)
 	pool->destroy(pool);
 	base->destroy(base);
 
-	/* due to size == 0 we get the requested IP back */
+	/* this results in an empty pool, which is rejected */
 	base = host_create_from_string("192.168.0.255", 0);
 	pool = mem_pool_create("test", base, 24);
-	ck_assert_int_eq(0, pool->get_size(pool));
-	assert_acquire(pool, "192.168.0.1", "192.168.0.1", MEM_POOL_NEW);
-	pool->destroy(pool);
-
+	ck_assert(!pool);
 	base->destroy(base);
 }
 END_TEST