MINOR: ncbuf: refactor ncb_advance()

MINOR: ncbuf: refactor ncb_advance()

First adjusted some typos in comments inside the function. Second,
change the naming of some variable to reduce confusion.

A special case has been inserted when advance is done inside a GAP block
and this block is the last of the buffer. In this case, the whole buffer
will be emptied, equivalent to a ncb_init() operation.