std.fs.File: fix discarding past end of file in positional mode

[torque]

std.Io.Reader.discardShort will loop infinitely if the read position is at the end of the file.

In std/fs/File.zig:

zig/lib/std/fs/File.zig

Lines 1382 to 1390 in 32a1aab

	.positional, .positional_reading => {
	const size = r.getSize() catch {
	r.mode = r.mode.toStreaming();
	return 0;
	};
	const delta = @min(@intFromEnum(limit), size - pos);
	r.pos = pos + delta;
	return delta;
	},

on L1387, if pos == size then delta == 0, so the function will return 0. Then in std/Io/Reader.zig

zig/lib/std/Io/Reader.zig

Lines 605 to 622 in 32a1aab

	pub fn discardShort(r: *Reader, n: usize) ShortError!usize {
	const proposed_seek = r.seek + n;
	if (proposed_seek <= r.end) {
	@branchHint(.likely);
	r.seek = proposed_seek;
	return n;
	}
	var remaining = n - (r.end - r.seek);
	r.seek = r.end;
	while (true) {
	const discard_len = r.vtable.discard(r, .limited(remaining)) catch |err| switch (err) {
	error.EndOfStream => return n - remaining,
	error.ReadFailed => return error.ReadFailed,
	};
	remaining -= discard_len;
	if (remaining == 0) return n;
	}
	}

on L615, discard_len will be 0, so remaining will never decrement.

A simple reproduction zig run repro.zig:

pub fn main() !void {
    const file = try std.fs.cwd().openFile("repro.zig", .{});
    var buffer: [4096]u8 = undefined;
    var reader = file.reader(&buffer);
    try reader.interface.discardAll(512);
}

const std = @import("std");

There are probably multiple ways to address this, and I don't know if this is the best one, but I am reasonably confident that infinite looping is not the correct behavior here.