std: Fix silent overflow in float parsing code
std: Fix silent overflow in float parsing code

A u64 can only hold 19 decimal digits, adjust the limit.

andrewrk/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig