Don't allow the pool allocator to be configured to allocate zero-sized objects

PR bootstrap/87747 would have been significantly easier to track down if
the pool allocator had faulted an attempt to configure it to allocate
zero-sized objects.  Instead, this slipped through and we later hit
memory corruption when the assumed size turned out to be different to
the configured size.

While, theoretically, there might be a use case for this, it seems
unlikely to me that GCC would have such a use.  So this patch adds a
checking assert that the object size is not zero.

	* alloc-pool.h (base_pool_allocator <TBlockAllocator>::initialize):
	Assert that the allocation size is not zero.

From-SVN: r265620