tdb: fix recovery reuse after crash

tdb: fix recovery reuse after crash

If a process (or the machine) dies after just after writing the
recovery head (pointing at the end of file), the recovery record will filled
with 0x42.  This will not invoke a recovery on open, since rec.magic
!= TDB_RECOVERY_MAGIC.

Unfortunately, the first transaction commit will happily reuse that
area: tdb_recovery_allocate() doesn't check the magic.  The recovery
record has length 0x42424242, and it writes that back into the
now-valid-looking transaction header) for the next comer (which
happens to be tdb_wipe_all in my tests).

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