Fixed timer and event race condition.

When idle timeout occurs at the same time as a request comes in,
the timer handler closes connection while the read event triggers
request processing, and this eventually leads to segmentation fault.

src/nxt_conn_write.c

@@ -22,9 +22,10 @@ nxt_conn_io_write(nxt_task_t *task, void *obj, void *data)
 
     c = obj;
 
-    nxt_debug(task, "conn write fd:%d", c->socket.fd);
+    nxt_debug(task, "conn write fd:%d er:%d bl:%d",
+              c->socket.fd, c->socket.error, c->block_write);
 
-    if (c->socket.error != 0) {
+    if (c->socket.error != 0 || c->block_write) {
         goto error;
     }