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

source/blender/compositor/intern/COM_MemoryProxy.cc

@@ -25,6 +25,7 @@ MemoryProxy::MemoryProxy(DataType datatype)
 {
   write_buffer_operation_ = nullptr;
   executor_ = nullptr;
+  buffer_ = nullptr;
   datatype_ = datatype;
 }