vacuum: fix crash on vacuum abort

vacuum: fix crash on vacuum abort

Martin Schwenke discovered that 517f05e42f17766b1e8db8f1f4789cbad968e304
("freeze: abort vacuuming when we're going to freeze.") used ctdb_db for
a logging message which is in fact uninitialized, causing a crash (even
if it wasn't actually logged).

Initialize it properly.  Also fix incorrect format in another logging
message introduced in that same change.

CQ:S1019093
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>