malloc/malloc.c: Avoid calling sbrk unnecessarily with zero

Due to my bad review suggestion for the fix for BZ #15089 a check
was removed from systrim to prevent sbrk being called with a zero
argument. Add the check back to avoid this useless work.

ChangeLog:

2014-06-19  Will Newton  <will.newton@linaro.org>

	* malloc/malloc.c (systrim): If extra is zero then return
	early.