intrusive_set_external_comparator: Fix _header having undefined color on move
intrusive_set_external_comparator: Fix _header having undefined color on move

swap_tree() doesn't change the color of the header, and becasue header
was not initialized, it is undefined (can be both red or black). One
problem this causes is that algo::is_header() expects the header to be
always red. It is used by unlink(), which for trees which have a black
header would infinite-loop.

The fix is to initialize the header.

Fixes #3242.

Message-Id: <e6310b53ca6ed3bfd87dba41454c6940ac6a5401@scylladb.com>

duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla