pg_dump: fix crash on error

If pg_dump fails to connect to Postgres, it attempts to print an error
message in ConnectDatabase():

/* check to see that the backend connection was successfully made */
if (PQstatus(AH->connection) == CONNECTION_BAD)
die_horribly(AH, modulename,
"connection to database \"%s\" failed: %s",
dbname, PQerrorMessage(AH->connection));

But if no database is explicitly specified, `dbname' is NULL, and libc
is entitled to crash if you pass a NULL pointer to it for a %s
formatting sequence (it actually does crash on Solaris, for example --
per report from Omar Kilani).

Attached is a patch that fixes this by just removing `dbname' from the
error message, as PQerrorMessage() should provide enough information.

Barring any objections, I'll apply this to HEAD and back branches within
24 hours.

-Neil