Fix segfault on OpenBSD. Do not touch the GTK page list during destruction.

Index: src/gui/docks/dockbook.cpp
--- src/gui/docks/dockbook.cpp.orig
+++ src/gui/docks/dockbook.cpp
@@ -105,6 +105,8 @@ DockBook::~DockBook()
 void
 DockBook::clear()
 {
+        if (deleting_) return;
+
 	// here the point: get_n_pages is fails because this is not notebook type
 	// i didn't know why this happens, possibly because clear() is called from destructor
 	// and 'this' is already deleted. Or, this function maybe never work right.
