pgsql: Don't Insert() a VFD entry until it's fully built.

Don't Insert() a VFD entry until it's fully built.

Otherwise, if FDDEBUG is enabled, the debugging output fails because
it tries to read the fileName, which isn't set up yet (and should in
fact always be NULL).

AFAICT, this has been wrong since Berkeley. Before 96bf88d52,
it would accidentally fail to crash on platforms where snprintf()
is forgiving about being passed a NULL pointer for %s; but the
file name intended to be included in the debug output wouldn't
ever have shown up.

Report and fix by Greg Nancarrow. Although this is only visibly
broken in custom-made builds, it still seems worth back-patching
to all supported branches, as the FDDEBUG code is pretty useless
as it stands.

Discussion: https://postgr.es/m/CAJcOf-cUDgm9qYtC_B6XrC6MktMPNRby2p61EtSGZKnfotMArw@mail.gmail.com

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d02dbbadb7cc7a3c70803c3b77a448b355ecb85b

Modified Files
--------------
src/backend/storage/file/fd.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)