fix mutt_buffy_notify() to properly update BuffyNotify

fix mutt_buffy_notify() to properly update BuffyNotify

In commit
184244053 doxy: document buffy.c
the check for whether to call mutt_buffy_list() was changed from
if (mutt_buffy_check(false) && BuffyNotify)
to
if (BuffyNotify && (mutt_buffy_check(false) != 0))

By reversing the check mutt_buffy_check() never executes when
BuffyNotify is zero. However, it is essential to run mutt_buffy_check()
before checking BuffyNotify, as mutt_buffy_check() updates BuffyNotify.

Therefore return the check to its inital form

Fixes #1279