[Bug libstdc++/63840] std::function copy constructor deletes an uninitialized pointer if new fails

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63840

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I thought this looked familiar, I already fixed the function(F) constructor for
PR 55320 but stupidly didn't check the copy constructor.