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.

ChangeLog

@@ -1,3 +1,8 @@
+2014-06-19  Will Newton  <will.newton@linaro.org>
+
+	* malloc/malloc.c (systrim): If extra is zero then return
+	early.
+
 2014-06-19  Siddhesh Poyarekar  <siddhesh@redhat.com>
 
 	* benchtests/Makefile ($(objpfx)bench-%.c): Remove $(.).

malloc/malloc.c

@@ -2749,6 +2749,9 @@ systrim (size_t pad, mstate av)
   /* Release in pagesize units, keeping at least one page */
   extra = (top_area - pad) & ~(pagesz - 1);
 
+  if (extra == 0)
+    return 0;
+
   /*
      Only proceed if end of memory is where we last set it.
      This avoids problems if there were foreign sbrk calls.