Registered RPC handlers affects port use counters.

Service process port gets closed and released by runtime on process exit,
but there can be active RPC handlers, which uses port structure.

src/nxt_port_rpc.c

@@ -143,6 +143,8 @@ nxt_port_rpc_register_handler_ex(nxt_task_t *task, nxt_port_t *port,
 
     nxt_debug(task, "rpc: stream #%uD registered", stream);
 
+    nxt_port_inc_use(port);
+
     return reg->data;
 }
 
@@ -332,6 +334,8 @@ nxt_port_rpc_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
     nxt_debug(task, "rpc: stream #%uD free registration", stream);
 
     nxt_mp_free(port->mem_pool, reg);
+
+    nxt_port_use(task, port, -1);
 }
 
 
@@ -421,6 +425,8 @@ nxt_port_rpc_remove_peer(nxt_task_t *task, nxt_port_t *port, nxt_pid_t peer)
         }
 
         nxt_mp_free(port->mem_pool, reg);
+
+        nxt_port_use(task, port, -1);
     }
 }
 
@@ -452,4 +458,6 @@ nxt_port_rpc_cancel(nxt_task_t *task, nxt_port_t *port, uint32_t stream)
     nxt_debug(task, "rpc: stream #%uD cancel registration", stream);
 
     nxt_mp_free(port->mem_pool, reg);
+
+    nxt_port_use(task, port, -1);
 }