passing fd over unix socket: fix a corner case where caller wants to pass no descriptors.
passing fd over unix socket: fix a corner case where caller
wants to pass no descriptors.

Previously the kernel would leak memory and try to free a potentially
arbitrary pointer.

Reviewed by:	pjd

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase