Improve r250890, so that we stop processing of a message with zero descriptors as early as possible, and assert that number of descriptors is positive in unp_freerights().
Improve r250890, so that we stop processing of a message with zero
descriptors as early as possible, and assert that number of descriptors
is positive in unp_freerights().

Reviewed by:	mjg, pjd, jilles

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