log: improve handling of use-before-init

Rather than segfaulting by attempting to traverse an initially
null log handler pointer, explicitly print a message and abort.

Signed-off-by: Matt Hoosier <matt.hoosier@gmail.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
[Pekka: coding style fix]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>