db: opening in non-read-only mode can fail depending how the path is specified

[RaduBerinde]

From testdata:

$ go run ../cmd/pebble db set db-stage-41 a a
error opening database at "db-stage-41": open : no such file or directory

$ go run ../cmd/pebble db set db-stage-41/ a a
(ok)

I think it has to do with the recent WAL changes. CC @jbowens