dlmalloc: Fix integer overflow in sbrk()

dlmalloc: Fix integer overflow in sbrk()

Make sure that the new break is within mem_malloc_start
and mem_malloc_end before making progress.
ulong new = old + increment; can overflow for extremely large
increment values and memset() can get wrongly called.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Simon Glass <sjg@chromium.org>