Although I tried to fix this earlier by introducing HALDEBUG_G(), it
turns out there seem to be other cases where the pointer value is still
NULL.
* Fix DO_HALDEBUG() and the HALDEBUG macro to check whether ah is NULL
before deferencing it
* Remove HALDEBUG_G() as it's no longer needed
This is hopefully a merge candidate for 9.0-RELEASE as enabling
debugging at startup could result in a kernel panic.