malloc_debug: round 0 byte allocations up to 1 byte

0 byte allocations can cause problems if they are immediately followed
by another allocation with no header, as both allocations will have the
same address.  Treat 0 byte allocations as 1 byte allocations so that
debug_iterate will return separate addresses for them.

Bug: 27578580
Change-Id: Ia8dc3481fa7062391e9b3ae58a36e8d47e7ee557