Reader.defaultReadVec: Workaround bad `r.end += r.vtable.stream()` behavior

If `r.end` is updated in the `stream` implementation, then it's possible that `r.end += ...` will behave unexpectedly. What seems to happen is that it reverts back to its value before the function call and then the increment happens. Here's a reproduction:

```zig
test "fill when stream modifies `end` and returns 0" {
    var buf: [3]u8 = undefined;
    var zero_reader = infiniteZeroes(&buf);

    _ = try zero_reader.fill(1);
    try std.testing.expectEqual(buf.len, zero_reader.end);
}

pub fn infiniteZeroes(buf: []u8) std.Io.Reader {
    return .{
        .vtable = &.{
            .stream = stream,
        },
        .buffer = buf,
        .end = 0,
        .seek = 0,
    };
}

fn stream(r: *std.Io.Reader, _: *std.Io.Writer, _: std.Io.Limit) std.Io.Reader.StreamError!usize {
    @memset(r.buffer[r.seek..], 0);
    r.end = r.buffer.len;
    return 0;
}
```

When `fill` is called, it will call into `vtable.readVec` which in this case is `defaultReadVec`. In `defaultReadVec`:

- Before the `r.end += r.vtable.stream` line, `r.end` will be 0
- In `r.vtable.stream`, `r.end` is modified to 3 and it returns 0
- After the `r.end += r.vtable.stream` line, `r.end` will be 0 instead of the expected 3

Separating the `r.end += stream();` into two lines fixes the problem (and this separation is done elsewhere in `Reader` so it seems possible that this class of bug has been encountered before).

Potentially related issues:

- https://github.com/ziglang/zig/issues/4021
- https://github.com/ziglang/zig/issues/12064

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