A pointer was checked for NULL after dereferencing it. The check is not needed here, except there's a bug which results in detaching the device twice.
A pointer was checked for NULL after dereferencing it. The check is not
needed here, except there's a bug which results in detaching the device
twice.

Move the NULL pointer check to the beginning of the function and convert
it into a KASSERT.

CID:		420
Found with:	Coverity Prevent(tm)
Discussed with:	ariff
MFC after:	5 days

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