qtdemux: Use safer clearing functions in _dispose()

In theory, dispose() functions should be idempotent and should be prepared not to crash or cause a double-free if an unref done from inside caused a recursive call to dispose() of the same object.

https://developer.gnome.org/gobject/stable/howto-gobject-destruction.html

This patch modifies the dispose() method to honor these constraints.

Since the double dispose() call won't actually occur in qtdemux (there is no cycle detection mechanism that could invoke it to work that way), this is more of a code cleanup than a user-facing problem fix.