Minor nit In FdSource::readUnbuffered

[dtzWill]

nix/src/libutil/serialise.cc

Line 136 in f5a2136

n = ::read(fd, (char *) data, bufSize);

Size used in read should be len (which we currently ignore), since were writing to data not our buffer.

Happily I think we only ever actually call this where data argument is the buffer (and len is it's size),
which would explain why this didn't break things at least sometimes xD.

Even so, this should probably get fixed anyway. Please 😇 (assuming folks agree with my analysis).