Fix a referenced before assignment in r193463

Some versions of Python on the builders seem strict about this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193464 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/lit/run.py

@@ -209,6 +209,7 @@ class Run(object):
         """
 
         # Choose the appropriate parallel execution implementation.
+        consumer = None
         if jobs != 1 and use_processes and multiprocessing:
             try:
                 task_impl = multiprocessing.Process