xfs_db: check for valid inode data pointer before dereferencing

xfs_db: check for valid inode data pointer before dereferencing

When processing an inode, the code checks various flags to determine
whether to output messages or not. When checking the CLI provided
inode numbers to be verbose about, we fail to check if the inode
data structre returned is valid or not before dereferencing it.
Hence running xfs_check with the "serious errors only" flag, xfs_db
will crash. Fix up the "should we output" logic to be safe.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>