Fix joystick user pointer NULL during disconnect

The joystick code did not distinguish between the allocation status of
the GLFW joystick object and whether it is connection to an OS level
joystick object.

These are now tracked separately.

Fixes #2092

src/internal.h

@@ -634,7 +634,8 @@ struct _GLFWmapping
 //
 struct _GLFWjoystick
 {
-    GLFWbool        present;
+    GLFWbool        allocated;
+    GLFWbool        connected;
     float*          axes;
     int             axisCount;
     unsigned char*  buttons;