BUG/MINOR: mworker: only match worker processes when looking for unspawned proc

BUG/MINOR: mworker: only match worker processes when looking for unspawned proc

In master-worker mode, when a freshly forked worker looks up its own
entry in proc_list to send its "READY" status to the master, the loop
was breaking on the first process with pid == -1 regardless of its
type. If a non-worker process (e.g. a master or program) also had
pid == -1, the wrong entry could be selected, causing send_fd_uxst()
to use an invalid ipc_fd.

Fix this by adding a PROC_O_TYPE_WORKER check to the loop condition,
and add a BUG_ON() assertion to catch any case where the loop exits
without finding a valid worker entry.

Must be backported to 3.1.