dlmalloc: Make sure allocation size is within malloc area

dlmalloc: Make sure allocation size is within malloc area

Since U-Boot does not support memory overcommit we can
enforce that the allocation size is within the malloc area.
This is a simple and efficient hardening measure to mitigate
further integer overflows in dlmalloc.

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