Let DDB's buf printer handle NULL pointers in the buf page array.

Let DDB's buf printer handle NULL pointers in the buf page array.

A buf's b_pages and b_npages fields may be inconsistent after a panic.
For instance, vfs_vmio_invalidate() sets b_npages to zero only after all
pages are unwired and their page array entries are cleared.

MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division