Load lines before nicklist

Nicklist is not nearly as important as the actual lines, load them first for better perceived performance. Parsing the nicklist can take a noticeable amount of time for channels with thousands of occupants.

This also improves/fixes the nicklist emptiness check
Fetch nicklist by buffer name, not pointer

An invalid pointer will crash WeeChat, while an invalid name will not. A pointer
becomes invalid e.g. if the buffer is closed by another client, g-b not updated,
and the buffer then selected in g-b.