[lit/Win] Check if a path was found before attempting to use it.

Summary:
This appears to break some bots, when getToolsPath fails to find some or
all of the tools (for example, an incomplete GnuWin32 installation).

Reviewers: zturner, modocache

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D38115

llvm-svn: 313854