[Bug demangler/80513] demangler walks past trailing nul in mangled name in a bunch of cases

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80513

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Richard Smith from comment #1)
> While we're here, this check for overflow in consume_count is nonsense, and
> any decent optimising compiler is going to optimise away the overflow check:
> 
> https://github.com/gcc-mirror/gcc/blob/master/libiberty/cplus-dem.c#L525
> 
> Testcase:
> 
> $ echo '_Z4294967297x' | c++filt
> x
> 
> Oops.

That overflow happens in d_number in cp-demangle.c, so that check isn't used
for your testcase. So we need to add a (not nonsense) check for overflow in
d_number, and fix the one in consume_count.