[AD] Change for exmouse to detect mouse

[Grzegorz Adam Hankiewicz <gradha@xxxxxxxxxx>]

Hi.

Today I upgraded my mouse to one of those wheel thingies, and playing
with it I discovered that exmouse crashes horribly if there's no mouse or
the devices don't have the correct read permissions, due to deferencing
mouse_driver, which is NULL. Proposed fix.

Index: exmouse.c
===================================================================
RCS file: /cvsroot/alleg/allegro/examples/exmouse.c,v
retrieving revision 1.4
diff -u -r1.4 exmouse.c
--- exmouse.c	14 May 2002 11:17:14 -0000	1.4
+++ exmouse.c	18 Sep 2002 20:14:49 -0000
@@ -22,7 +22,6 @@
    if (allegro_init() != 0)
       return 1;
    install_keyboard(); 
-   install_mouse();
    install_timer();
    if (set_gfx_mode(GFX_SAFE, 320, 200, 0, 0) != 0) {
       set_gfx_mode(GFX_TEXT, 0, 0, 0, 0);
@@ -33,6 +32,14 @@
    text_mode(makecol(255, 255, 255));
    clear_to_color(screen, makecol(255, 255, 255));
 
+   /* Detect mouse presence */
+   if (install_mouse() < 0) {
+      textout_centre(screen, font, "No mouse detected, but you need one!",
+	 SCREEN_W/2, SCREEN_H/2, makecol(0, 0, 0));
+      readkey();
+      return 0;
+   }
+   
    textprintf_centre(screen, font, SCREEN_W/2, 8, makecol(0, 0, 0),
 		     "Driver: %s", mouse_driver->name);