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

gcc/ChangeLog

@@ -1,3 +1,8 @@
+2018-10-30  Richard Earnshaw  <rearnsha@arm.com>
+
+	* alloc-pool.h (base_pool_allocator <TBlockAllocator>::initialize):
+	Assert that the allocation size is not zero.
+
 2018-10-30  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/87800

gcc/alloc-pool.h

@@ -256,6 +256,7 @@ base_pool_allocator <TBlockAllocator>::initialize ()
   size_t size = m_size;
 
   gcc_checking_assert (m_name);
+  gcc_checking_assert (m_size);
 
   /* Make size large enough to store the list header.  */
   if (size < sizeof (allocation_pool_list*))