write_vc: fix write retry loop for nonblocking mode

This is a simple fix for the write retry loop that is used on
non-blocking connections if write() failed with -EAGAIN.

Additionally it removes a redundant if () {}

Erroneously at each cycle of the loop the length of the data
to send is incremented and the buffer pointer is decremented.
Thus, it might happen that:
* the application crashes
* data from the memory before the buffer is sent

Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Steve Dickson <steved@redhat.com>