tipc: Minor optimization to topology service connection establishment

Eliminates a local iovec structure containing no data, which was
previously used during the establishment of a topology service connection,
since the same effect can be achieved by passing in a NULL pointer and
an iovec length of zero.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>