Fix infinite loop in MemReader::read_until.

[qpliu]

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.

[sfackler]

It actually looks like all implementations of Buffer return an empty buffer on EOF. @alexcrichton: what's the intended API here?

[alexcrichton]

This looks like the correct fix to me. I intended it to not be an error if fill returned a slice of length 0. The BufferedReader implementation can pass through 0-length reads because of this.

This was definitely in error though, because you have genuinely hit an error at the end of a MemReader or BufReader

[sfackler]

Cool. @qpliu could you implement the same fix for BufReader and BufferedReader?

[alexcrichton]

I think that BufferedReader should be ok (may need some tests though) because the underlying reader will eventually return EOF. BufReader, however, likely needs this fix.