Fix compile warnings under GCC 15.2.1

[ndossche]

The compiler now infers constness through string functions. Adhere to that.
It also now warns about using multiplications in conditions.

[iluuu1994]

We usually get reports for stable branches when new compiler versions become generally available, should be backport immediately or wait until that happens?

[iluuu1994]

I don't understand this cast, why is it necessary?

[ndossche]

ptr is a const char * but end is a char *.
end cannot be made const because it is written to on line 1780. At that point, ptr actually isn't pointing to a constant character buffer anymore.
I didn't want to do too many changes because to fix this nicely we'd need to split the ptr variable.

[iluuu1994]

Oh ok, so the return type of strchr is now effectively const-generic. Right, it does seem like it would be best to split the vars. I'd prefer that, at least for master.

[ndossche]

We usually get reports for stable branches when new compiler versions become generally available, should be backport immediately or wait until that happens?

This is the compiler that has been shipping for a few weeks now on Arch/Manjaro. So I'd say we go ahead with doing it directly in 8.4.