[client] Use a QPointer for the enteredSurface in Pointer

If the client deletes the currently entered Surface the Pointer should
return null in Pointer::enteredSurface and not invalid memory.

At the same time a now incorrect assert is removed. After the client
deletes the Surface it might still get a left event for the Surface.
In that case a comparison between the enteredSurface and the Surface
from the leave event would fail.

Reviewed-By: bshah