ion: Fix array-out-of-bounds in system heap error path

After creating all secure pools successfully, i == VMID_LAST.
If ion_system_heap_create_pools(), the error path will attempt to
destroy all secure pools, starting from i.

But this is one past the end of the array.
struct ion_page_pool **secure_pools[VMID_LAST];

CRs-Fixed: 997730
Change-Id: Id97ab40a1dc0885f476f6e25cdd324f5ee3dcf04
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>