mpegts: Don't confuse slice allocator with regular one

Previously selector_bytes and private_data_bytes were sometimes allocated and
free using the normal allocator and sometimes using the slice allocator.
Additionally prefer g_strdup() to g_memdup() for strings.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732789
mpegts: No need to check for NULL before calling g_free()

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732789
mpegts: Plug leak allocated using slice allocator

GstMpegtsDVBLinkageDescriptor was not always freed. Complete the
existing error handling and free the allocations before returning.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732789