Handle NULL address argument in the `mach_vm_[de]allocate()`
interceptors and fix test: `Assignment 2` is not valid if we weren't
able to re-allocate memory.
rdar://67680613
I verified that the test is red without the interceptors.
rdar://40334350
Reviewed By: kubamracek, vitalybuka
Differential Revision: https://reviews.llvm.org/D66616
llvm-svn: 371439