Fix typo(-ish)

The «if err» test always passed after #cf938e99926 changed the earlier
assignment to always set err to "\n" if stderr was empty, and so every
script plugin (e.g., ec2.py) started to always report an empty ERROR.

lib/ansible/plugins/inventory/script.py

@@ -121,7 +121,7 @@ class InventoryModule(BaseInventoryPlugin, Cacheable):
                     raise AnsibleError("failed to parse executable inventory script results from {0}: {1}\n{2}".format(path, to_native(e), err))
 
                 # if no other errors happened and you want to force displaying stderr, do so now
-                if err and self.get_option('always_show_stderr'):
+                if stderr and self.get_option('always_show_stderr'):
                     self.display.error(msg=to_text(err))
 
             processed = self._cache[cache_key]