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.
24 jobs for qtdemux-dispose in 63 minutes and 44 seconds (queued for 4 seconds)