Use sock->nchildren instead of mgr->nworkers when initializing NM

On Windows, we were limiting the number of listening children to just 1,
but we were then iterating on mgr->nworkers.  That lead to scheduling
more async_*listen() than actually allocated and out-of-bound read-write
operation on the heap.

lib/isc/netmgr/netmgr-int.h

@@ -720,7 +720,7 @@ struct isc_nmsocket {
 
 	/*% Child sockets for multi-socket setups */
 	isc_nmsocket_t *children;
-	int nchildren;
+	uint_fast32_t nchildren;
 	isc_nmiface_t *iface;
 	isc_nmhandle_t *statichandle;
 	isc_nmhandle_t *outerhandle;