patch 9.0.1992: [security] segfault in exmode

Problem:  segfault in exmode when redrawing
Solution: skip gui_scroll when exmode_active

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/gui.c

@@ -4397,6 +4397,7 @@ gui_do_scrollbar(
  * Scroll a window according to the values set in the globals
  * "current_scrollbar" and "scrollbar_value".
  * Return TRUE if the cursor in the current window moved or FALSE otherwise.
+ * may eventually cause a redraw using updateWindow
  */
     int
 gui_do_scroll(void)
@@ -4416,6 +4417,9 @@ gui_do_scroll(void)
     if (wp == NULL)
 	// Couldn't find window
 	return FALSE;
+    // don't redraw, LineOffset and similar are not valid!
+    if (exmode_active)
+	return FALSE;
 
     /*
      * Compute number of lines to scroll.  If zero, nothing to do.