Bug fix.

Bug fix.

When all the servers for a given server go away, the router deletes the
server class from its internal data structures.  However that can happen
in the middle of a loop receiving successive messages from that server.

The old code would continue trying to read more messages from the
deleted server class, leading to a segfault.

The new code checks to see whether the server class still exists.  If
not, it breaks out of the loop.

M    src/router/osrf_router.c

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1899 9efc2488-bf62-4759-914b-345cdb29e865