[Bug libstdc++/110862] format out of bounds read on format string "{0:{0}"

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110862

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://gcc.gnu.org/git?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/std/format;h=9d5981e4882991cc2cbfb9353d399372030e8722;hb=refs/heads/master#l3535
needs to throw if begin()==end() || *begin() != '}'

Fix incoming as soon as I have a decent WiFi signal.