Chain of two large (i.e. BytesMut) panics

[jasl8r]

When chaining two spacious buffers, as in unbounded BytesMut, the remaining and remaining_mut fn both panic due to overflow when adding together the two child remaining. Obviously BytesMut acts as though it is infinite and this happens. I don't know if I'm dumb for chaining two infinite buffers, but it was just the simple way for me to start testing out Bytes and a Chain. The fix is of course simple, just change the unwrap to unwrap_or(usize::MAX).

[taiki-e]

This will be fixed by #488.

[taiki-e]

#488 merged