[Bug libstdc++/87787] [9 Regression][UBSAN] runtime error: null pointer passed as argument 2, which is declared to never be null

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87787

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I want to check Marc's comment 2 observations first.