BigSwitch: Move attr ref after error check

Change in Big Switch server manager module:
Moves an attribute reference to a line after
the error check that validates the object is
not None.

Closes-Bug: #1298699
Change-Id: I5f9abf3b456d5066e90c05bc6b3aa5adcecb7943
Merge "BigSwitch: Move attr ref after error check"