Previously, our use of abort() would break anywhere that we didn't
include stdlib.h.  This was especially troublesome in case where
tor_assert_nonfatal() was used with ALL_BUGS_ARE_FATAL, since that
one seldom gets tested.

As an alternative, we could have just made this header include
stdlib.h.  But that seems bloaty.

Fixes bug 30189; bugfix on 0.3.4.1-alpha.