Fix for "argument must be an int, or have a fileno() method" error

The issue was that, when forks == 1, the _executor() function was
being called with None for the value of new_stdin.

Fixes #3841, #3902

lib/ansible/runner/__init__.py

@@ -363,6 +363,9 @@ class Runner(object):
             return flags
 
         try:
+            if not new_stdin:
+                self._new_stdin = os.fdopen(os.dup(sys.stdin.fileno()))
+            else:
                 self._new_stdin = new_stdin
 
             exec_rc = self._executor_internal(host, new_stdin)