Be consistent with setting and testing for breathing_space

Be consistent with setting and testing for breathing_space

	* src/alloc.c (refill_memory_reserve): Test if breathing_space is
	NULL, not zero.
	Don't cast the malloc.

	* src/alloc.c (garbage_collect_1): Increase breathing_space to,
	erm, give us more breathing space.

	* src/alloc.c (reinit_alloc_once_early): Set breathing_space to
	NULL, not zero.

Signed-off-by: Steve Youngs <steve@sxemacs.org>