vknulldrv: Use a default allocator when none provided

The loader used to always provide an allocator, even if the app
didn't. It no longer does that, so the null driver must be prepared to
provide its own default allocator.

Fixes bug 28303142

Change-Id: I6ab946913dc4edaf2a0b07ab9ae8c43be69b3ed6
1 file changed