more gracefully account for undefined stdout

see: https://github.com/ansible/tower/issues/1215
related: https://github.com/ansible/tower/pull/1192#issuecomment-377982131

awx/main/tasks.py

@@ -885,6 +885,7 @@ class BaseTask(LogErrorsTask):
         output_replacements = []
         extra_update_fields = {}
         event_ct = 0
+        stdout_handle = None
         try:
             kwargs['isolated'] = isolated_host is not None
             self.pre_run_hook(instance, **kwargs)