[Bug pch/116936] [UBSAN] gcc/diagnostic.cc: null pointer passed as argument

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116936

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 59268
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59268&action=edit
gcc15-pr116936.patch

Untested fix.