Fix vector memory statistics.

Fix vector memory statistics.

2018-11-05  Martin Liska  <mliska@suse.cz>

	* mem-stats.h (mem_alloc_description::release_instance_overhead):
	Return T *.
	* vec.c (struct vec_usage): Register m_element_size.
	(vec_prefix::register_overhead): New arguments: elements and
	element_size.
	(vec_prefix::release_overhead): Subtract elements.
	* vec.h (struct vec_prefix): Change signature.
	(va_heap::reserve): Pass proper arguments.
	(va_heap::release): Likewise.

From-SVN: r265799