Fixed segmentation fault during reconfiguration.

If idle connection was closed before h1proto had been allocated
then c->socket.data is NULL.  This happens if nxt_h1p_idle_response()
is called by nxt_h1p_idle_close().  However, h1p->conn_write_tail
is used only in nxt_h1p_request_send() that would not be called
after nxt_h1p_idle_response().

The bug was introduced in f237e8c553fd.

src/nxt_h1proto.c

@@ -1876,7 +1876,6 @@ nxt_h1p_idle_response(nxt_task_t *task, nxt_conn_t *c)
     u_char     *p;
     size_t     size;
     nxt_buf_t  *out, *last;
-    nxt_h1proto_t  *h1p;
 
     size = nxt_length(NXT_H1P_IDLE_TIMEOUT)
            + nxt_http_date_cache.size
@@ -1906,9 +1905,6 @@ nxt_h1p_idle_response(nxt_task_t *task, nxt_conn_t *c)
     last->completion_handler = nxt_h1p_idle_response_sent;
     last->parent = c;
 
-    h1p = c->socket.data;
-    h1p->conn_write_tail = &last->next;
-
     c->write = out;
     c->write_state = &nxt_h1p_timeout_response_state;