Fix failure in get_filecheck_path when filecheck is None

If the path was not specified (and it's None), lexists throws an exception rather than returning False. get_filecheck_path now checks whether filecheck is set before calling lexists

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@344410 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed