If we get an exception when getting the southbound client in method
get_device_mapping_from_network we will end up seeing the following
error:
UnboundLocalError: local variable 'conn' referenced before assignment
This patch fixes this issue by making sure that the conn variable exists
even if that call fails.
TrivialFix
Closes-Bug: #1888550
Change-Id: I621a00d34e9c2b14234e11e94857a39fe54a111f