[Bug libstdc++/87872] debug list::splice should not call _M_transfer_from_if on self-splices

[jbytheway at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87872

--- Comment #1 from John Bytheway <jbytheway at gmail dot com> ---
On further reflection, it would make more sense to put this check inside
_M_transfer_from_if, rather than in every splice function.