It is possible that the seek head at the front has been removed due to
moving the seek head at the back one byte to the front which calls
remove_from_meta_seeks() with that second seek head. This leads to the
first seek head being empty and therefore being removed.
Later calls to add_to_meta_seek() must therefore make sure that, even if
the element could be added to an existing seek head at the end, a new
seek head is created at the start referencing the one at the end.
Fixes#1513.
