ion: return proper error value on allocation failure

Return ENOMEM on allocation failure from ion_system_heap_allocate.
The absence of this was resulting in ion_buffer_create to proceed
further and thus causing a crash.

Change-Id: I906b129b5fe1c74f7176116142085fbb47c3c834
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>