Fixing Coverity warnings.

CID 200496
CID 200494
CID 200490
CID 200489
CID 200483
CID 200482
CID 200472
CID 200465

src/nxt_main_process.c

@@ -808,6 +808,9 @@ nxt_main_cleanup_worker_process(nxt_task_t *task, nxt_pid_t pid)
 
                 buf = nxt_buf_mem_ts_alloc(task, task->thread->engine->mem_pool,
                                            sizeof(pid));
+
+                nxt_assert(buf != NULL);
+
                 buf->mem.free = nxt_cpymem(buf->mem.free, &pid, sizeof(pid));
 
                 nxt_port_socket_write(task, port, NXT_PORT_MSG_REMOVE_PID,