error when entering main loop with no windows defined, as GLUT does

git-svn-id: https://svn.code.sf.net/p/freeglut/code/trunk@1552 7f0cb862-5218-0410-a997-914c9d46530a

freeglut/freeglut/progs/demos/CallbackMaker/CallbackMaker.c

@@ -227,6 +227,7 @@ Error(const char *fmt, va_list ap)
 
     /* print warning message */
     vprintf(fmt, ap);
+    printf("\n");
 
     /* terminate program, after pause for input so user can see */
     printf ( "Please enter something to exit: " );

freeglut/freeglut/src/fg_main.c

@@ -414,6 +414,9 @@ void FGAPIENTRY glutMainLoop( void )
 
     FREEGLUT_EXIT_IF_NOT_INITIALISED ( "glutMainLoop" );
 
+    if (!fgStructure.Windows.First)
+        fgError(" ERROR:  glutMainLoop called with no windows created.");
+
 	fgPlatformMainLoopPreliminaryWork ();
 
     fgState.ExecState = GLUT_EXEC_STATE_RUNNING ;