add NULL init for frontprim and backprim (not necessary, but some compiler complain about this)

gl2ps.c

-/* $Id: gl2ps.c,v 1.207 2005-04-27 19:01:43 geuzaine Exp $ */
+/* $Id: gl2ps.c,v 1.208 2005-05-02 15:59:49 geuzaine Exp $ */
 /*
  * GL2PS, an OpenGL to PostScript Printing Library
  * Copyright (C) 1999-2005 Christophe Geuzaine <geuz@geuz.org>
@@ -1208,7 +1208,7 @@ static GLboolean gl2psLess(GLfloat f1, GLfloat f2)
 
 static void gl2psBuildBspTree(GL2PSbsptree *tree, GL2PSlist *primitives)
 {
-  GL2PSprimitive *prim, *frontprim, *backprim;
+  GL2PSprimitive *prim, *frontprim = NULL, *backprim = NULL;
   GL2PSlist *frontlist, *backlist;
   GLint i, index;