[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

llvm/utils/lit/lit/llvm/config.py

@@ -27,7 +27,8 @@ class LLVMConfig(object):
             path = self.lit_config.getToolsPath(config.lit_tools_dir,
                                            config.environment['PATH'],
                                            ['cmp.exe', 'grep.exe', 'sed.exe'])
-            self.with_environment('PATH', path, append_path=True)
+            if path is not None:
+                self.with_environment('PATH', path, append_path=True)
             self.use_lit_shell = True
 
         # Choose between lit's internal shell pipeline runner and a real shell.  If