Fix infinite loop in MemReader::read_until.
Fix infinite loop in BufReader::read_until.
auto merge of #12106 : qpliu/rust/memreader-fill-eof, r=sfackler

I don't know if anything depends on MemReader::fill returning an empty slice instead of EndOfFile, but I'm pretty sure that MemReader::read_until should not go into an infinite loop.