expr: increase size of varname

[SuperSamus]

1 dollar sign + 1 negative sign + 10 characters for the longest number + 1 null terminator = 13 characters.
Fixes compilation error with certain flags.

[Pokechu22]

Looks good to me. You might want to send this upstream to https://github.com/zserge/expr too (though I'm not sure how actively that's maintained).

CC @smurf3tte, @TryTwo since you originally worked on conditional breakpoints and are more familiar with this code.

[SuperSamus]

It's 4 there, for some reason.

[Pokechu22]

Looks like that was changed in the first force-push on #9334 (https://github.com/dolphin-emu/dolphin/compare/011ce338d8b190330b35a8361e161ad968a82bba..01f5d4774bff6171cfa0bf575323bfc625ee6de3). I don't see any corresponding review comments though.

It looks like the original code used sizeof(varname) - 1 in addition to using a 4-element array. Since snprintf writes "At most buf_size - 1 characters" in addition to the null terminator, that means that the original implementation only works for up to 10 macro parameters (after which it'd start assigning duplicates, I guess?). In practice, that's probably all that's ever needed, and I think neither billions of macro parameters nor a negative number of macro parameters makes sense in practice, but it's still better to be safe (and to fix the warning).

[TryTwo]

I'm not sure. I'd have to do some furious breakpointing in VS to make sense of it. If bumping it by one isn't expected to break anything, I guess it's okay. If you need someone to look into it, let me know.

[smurf3tte]

The index in question can't be negative, so the only real effect of this change is to silence an erroneous warning.