Kernel.Vmm: Fix potential race condition involving concurrent Allocate and Free calls (#3978)

* Avoid nullptr dereference on GetSocket

Was gonna include this in my socket PR, but that got merged before I could push this.

* Lock unmap mutex in PoolExpand and Allocate

PAYDAY 2 has a rare race condition involving dmem releases.
I'm not certain this commit will fix it, but this would cause a race condition that could cause asserts like what PAYDAY 2 can hit, so I'll just pray this does the job until I can prove it doesn't.