tui: Simplify tui_alloc_content

tui: Simplify tui_alloc_content

I stumbled upon this while doing some cxx-conversion work.  Since the
x-family alloc functions throw on failure, it is useless to test their
result for failure.  The else branch of != NULL is basically dead code.

I changed the type of element_block_ptr to struct tui_win_element, which
seems obvious (this is actually what raised the flag, casting the result
of xmalloc to struct tui_win_element* wouldn't work).

gdb/ChangeLog:

	* tui/tui-data.c (tui_alloc_content): Don't check xmalloc
	result.  Change type of element_block_ptr.  Change allocation to
	use XNEWVEC.