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

[pavel.kryukov at phystech dot edu via Gcc-bugs]

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

--- Comment #5 from Pavel I. Kryukov <pavel.kryukov at phystech dot edu> ---
> Are you willing and able to certify the origin of the code under the terms of 
> https://gcc.gnu.org/dco.html ?

Yes. Do I need to resubmit a signed-off patch?