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Initialize the fourth and final instance variable of MemoryProxy

The constructor of MemoryProxy initializes 3 of 4 instances variables.
If a MemoryProxy is constructed and MemoryProxy::free is called
on this instance, buffer_ is undefined and 'delete buffer_;' causes errors.

Although this misuse pattern does not exist in the current codebase
it already tripped up the Address Sanitizer on various occasions
while debugging unrelated problems.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D13569