[sanitizer] Minor refactor of some ReservedAddressRange functions

Summary:
Some of the functions had spurious conditional statements and checks, and some
intermediary variables that I feel made the code more complicated than it needs
to be. Also, when unmapping the whole range, the range size would be 0, but
the base was set to the address of the end of the range, which sounds prone to
error. I think nulling out the base in this scenario is a better way to go.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45775

llvm-svn: 330355