dashdemux: Cleanup memory allocation/deallocation

dashdemux: Cleanup memory allocation/deallocation

Ensure that g_free/xmlFree is used correctly based on how the
memory was allocated.

When deallocating GLists, there were many places that were using
g_list_foreach and g_list_free.  Converted these occurrences to
call g_list_free_full.

Add NULL checks to all xmlFree calls since the documentation does
not guarantee that passing NULL is safe

In places where we are strdup'ing memory allocated by libxml2,
changed those calls to use xmlMemStrdup().

There were several places where we were missing g_slice_free when
deallocating a top-level node structure.

https://bugzilla.gnome.org/show_bug.cgi?id=702837