bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > - I did not see that l_l_o_p is set to false anywhere.  Should it be? 
> > What if a buffer is modified in such a way that there are no long lines 
> > anymore?
>
> Gosh!  That was an error, thanks!

In fact it wasn't, there is a "b->long_line_optimizations_p = 0;" in 
reset_buffer.