ovn-nbctl: Always initialize output arguments in *_by_name_or_uuid().

ovn-nbctl: Always initialize output arguments in *_by_name_or_uuid().

This makes it easier to reason about the outputs, and fixes warnings for
GCC 6.3.x.

Reported-by: Ian Stokes <ian.stokes@intel.com>
Cc: Venkata Anil <vkommadi@redhat.com>
Fixes: 31114af758c7 ("ovn-nbctl: Update logical router port commands.")
Fixes: 80f408f4cffb ("ovn: Use Logical_Switch_Port in NB.")
Fixes: 36f232bca2db ("ovn: l3ha, CLI for logical router port gateway chassis")
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ian Stokes <ian.stokes@intel.com>