UBI: fix out of bounds write

If aeb->len >= vol->reserved_pebs, we should not be writing aeb into the
PEB->LEB mapping.

Caught by Coverity, CID #711212.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>