dump: Fix for db

[emaste]

This ends up calling into ffs_sbget which initializes it like so:

int
ffs_sbget(void *devfd, struct fs **fsp, off_t altsblock,
    struct malloc_type *filltype,
    int (*readfunc)(void *devfd, off_t loc, void **bufp, int size))
{
...
        *fsp = NULL;