textview: Fix comparison thinko

Apparently, we have no test that checks for actually drawing text, so
the thinko that caused all text to disappear in
4b07d28153 went unnoticed.

gtk/gtktextlayout.c

@@ -723,7 +723,7 @@ gtk_text_layout_get_lines (GtkTextLayout *layout,
 
   g_return_val_if_fail (GTK_IS_TEXT_LAYOUT (layout), NULL);
 
-  if (top_y <= bottom_y)
+  if (top_y >= bottom_y)
     return NULL;
 
   retval = NULL;