multifilesink: don't leak memory when no max-files limit is set

multifilesink: don't leak memory when no max-files limit is set

Technically we weren't leaking the memory, just storing it internally
and never using it until the element is freed. But we'd still use more
and more memory over time, so this is not good over longer periods
of time. Only keep track of files if there's actually a limit set,
so that we will prune the list from time to time.

https://bugzilla.gnome.org/show_bug.cgi?id=766991