Fix bug 9122: don't allow newdefaultoptions to be NULL

(This caused a crash that was reported as bug 9122, but the underlying
behavior has been wrong for a while.)

Fix on 0.2.3.9-alpha.
Give a warning when bufferevents are enabled.

Ticket 9147.
Merge branch 'bug9147' into maint-0.2.4
Merge branch 'maint-0.2.4' into release-0.2.4