Fix thinko in commit 8be8510c: it's okay to have dbid == 0 in normal
(non-pin) entries in pg_shdepend, because global objects such as
databases are entered that way.  The test would pass so long as it
was run in a cluster containing no databases/tablespaces owned by,
or granted to, roles other than the bootstrap superuser.  That's the
expected situation for "make check", but for "make installcheck", not
so much.

Reported by Ryan Murphy.

Discussion: https://postgr.es/m/CAHeEsBc6EQe0mxGBKDXAwJbntgfvoAd5MQC-5362SmC3Tng_6g@mail.gmail.com