[Bug libstdc++/103853] std::forward_list::merge should check if __list != this

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103853

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Pavel, I'm going to use your original patch, or parts of it anyway (the
addition to the changelog is wrong, we do not patch the changelog by hand it's
auto-generated).

Are you willing and able to certify the origin of the code under the terms of
https://gcc.gnu.org/dco.html ?