UPBGE: Fix mathutils callbacks registration.

The assertion in Mathutils_RegisterCallback was raised when all the callbacks
were registered (freestyle and bmesh too).
But this assertion introduced in 1b74ec9 is wrong. The test i + 1 < MAX
suppose that the last callback is a null sentinel. But this is not the
case as no one is using this to stop a loop and the loop in
Mathutils_RegisterCallback must stop when it find a null slot or that the
slot is already containing the callback, otherwise it's an error.
It's impossible to stop the loop with all regular slots registered and
and different from the one intenting to register.

Fix a part of issue #785.

source/blender/python/mathutils/mathutils.c

@@ -442,7 +442,7 @@ unsigned char Mathutils_RegisterCallback(Mathutils_Callback *cb)
 			return i;
 	}
 
-	BLI_assert(i + 1 < MATHUTILS_TOT_CB);
+	BLI_assert(i < MATHUTILS_TOT_CB);
 
 	mathutils_callbacks[i] = cb;
 	return i;