Small bug fix: the condition in the final test at the end of
read_stream_look_ahead() wasn't quite right. In general when looking
ahead, we don't need to start a read just because the pending read
would bring us up to stream->distance if submitted now (we'd prefer to
build it all the way up to size io_combine_limit if we can), but if
that condition is met AND we have nothing pinned yet, then there is no
chance for the read to grow bigger by a pinned buffer being consumed.
Fixed, comment updated.
