patch 9.0.0321: cannot use the message popup window directly

Problem:    Cannot use the message popup window directly.
Solution:   Add ":echowindow".

src/popupwin.c

@@ -1302,7 +1302,8 @@ popup_adjust_position(win_T *wp)
 	}
 	if (wp->w_popup_pos == POPPOS_BOTTOM)
 	    // assume that each buffer line takes one screen line
-	    wp->w_winrow = MAX(Rows - wp->w_buffer->b_ml.ml_line_count - 1, 0);
+	    wp->w_winrow = MAX(cmdline_row
+				    - wp->w_buffer->b_ml.ml_line_count - 1, 0);
 
 	if (!use_wantcol)
 	    center_hor = TRUE;