Check the glutDisplayFunc() callback for being a NULL pointer. This is

illegal and is disallowed in GLUT 3.0+, so we disallow it too in
freeglut 2.0.1+


git-svn-id: https://svn.code.sf.net/p/freeglut/code/trunk@221 7f0cb862-5218-0410-a997-914c9d46530a

freeglut/freeglut/src/freeglut_callbacks.c

@@ -48,6 +48,9 @@
  */
 void FGAPIENTRY glutDisplayFunc( void (* callback)( void ) )
 {
+    if( !callback )
+	fgError ("Fatal error in program.  NULL display callback not "
+	    "permitted in GLUT 3.0+ or freeglut 2.0.1+\n");
     SET_CALLBACK( Display );
 
     /*