Fix return value call on send() when sending NetFlow messages.

Fix return value call on send() when sending NetFlow messages.

When sending NetFlow messages, we use the send() call, but were checking
the wrong return value.  It would report an error when any non-zero
value was returned.  The send() call returns the number of bytes sent or
-1 on error.  Thus, whenever a NetFlow message was sent, it would
generate an error message.  Now, we only log a message when a value of
-1 is returned.  (Bug #1166)