Make BufferedReader propagate 0-byte long reads.

[spaolacci]

Could prevent callers from catching the situation and lead to e.g early
iterator terminations (cf. Reader::read_byte) since None is only to
be returned only on EOF.

[alexcrichton]

I don't really understand why the check for an empty buffer moved lower, it seems like the condition you want is to have if available.len() == 0 && self.inner.reader.eof() { return None; }.

Additionally, this is a tricky case which definitely needs a test.

[spaolacci]

I definitely agree with the test requirement. The reason for moving down the condition is (/was) to avoid multi-borrowing of (*self).inner. I honestly didn't find any easier way to bypass the following situation

buffered.rs:121:39: 121:49 error: cannot borrow `(*self).inner` as mutable more than once at a time
buffered.rs:121             if available.len() == 0 && self.inner.eof() {
                                                       ^~~~~~~~~~
buffered.rs:120:28: 120:33 note: second borrow of `(*self).inner` as mutable occurs here
buffered.rs:120             let available = self.fill();
                                            ^~~~~

I'm quite new to the language (to say the least), so I'm obviously open to any lead/suggestion with doing it any other way (or with just opening a bug and letting the patch down to not make you loose too much of your time).

[alexcrichton]

Ah I can see where that would be a problem, r+ with a test!