Fix in_availableJoysticks cvar not updating

When in_restart is called or SDL controller added/remove event is
received, IN_InitJoystick() tried to update the list but instead
of updating the cvar Cvar_Get() just printed a developer message:

  Warning: cvar "in_availableJoysticks" given initial values:
  "first joystick
  ", "first joystick
  second joystick
  "

code/sdl/sdl_input.c

@@ -503,7 +503,10 @@ static void IN_InitJoystick( void )
 		Q_strcat(buf, sizeof(buf), "\n");
 	}
 
-	Cvar_Get( "in_availableJoysticks", buf, CVAR_ROM );
+	Cvar_Get( "in_availableJoysticks", "", CVAR_ROM );
+
+	// Update cvar on in_restart or controller add/remove.
+	Cvar_Set( "in_availableJoysticks", buf );
 
 	if( !in_joystick->integer ) {
 		Com_DPrintf( "Joystick is not active.\n" );