isa_bus_irqs(), isa_create() and isa_try_create() call hw_error() when
passed a null bus. Use of hw_error() has always been questionable,
because these are used only during machine initialization, and
printing CPU registers isn't useful there.
Since the previous commit, passing a null bus is a programming error.
Drop the hw_error() and simply let it crash.
Cc: Richard Henderson <address@hidden>
Cc: "Michael S. Tsirkin" <address@hidden>
Cc: "Hervé Poussineau" <address@hidden>
Cc: Aurelien Jarno <address@hidden>
Cc: Mark Cave-Ayland <address@hidden>
Signed-off-by: Markus Armbruster <address@hidden>
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