gtktextview: Fixed arrow key crash.

When pressing the keyboard arrows to move around when the insertion point is
hidden, it causes an assertion error in blink_cb.

Insertion point blinks should only be scheduled when blinking is enabled and the
insertion point is visible.

Closes #4275

gtk/gtktextview.c

@@ -6202,7 +6202,7 @@ gtk_text_view_check_cursor_blink (GtkTextView *text_view)
 static void
 gtk_text_view_pend_cursor_blink (GtkTextView *text_view)
 {
-  if (cursor_blinks (text_view))
+  if (cursor_blinks (text_view) && cursor_visible (text_view))
     {
       remove_blink_timeout (text_view);
       add_blink_timeout (text_view, TRUE);