MI Refactor CMIUtilSystemWindows::GetExecutablesPath()

Summary:
My understanding of the Windows API call GetLastError() is that it should only be checked when ::GetModuleFileName()  returns 0 on error.
Otherwise GetExecutablesPath() could return an error despite nLen being valid if GetLastError() was inconsistent.
Patch updates function to only call GetOSLastError() when nLen == 0

Patch from ewan@codeplay.com

Reviewers: EwanCrawford

Subscribers: lldb-commits, deepak2427

Differential Revision: http://reviews.llvm.org/D9154

llvm-svn: 235515