Re: Fix a server crash problem from pg_get_database_ddl

I have reproduced this error against the current master:

```

CREATE TABLESPACE ts1 LOCATION '/workspace/tablespaces/pg_bug_ts1';

CREATE DATABASE db1 TABLESPACE ts1;

DELETE FROM pg_tablespace WHERE spcname = 'ts1';

SELECT * FROM pg_get_database_ddl('db1'::regdatabase);

server closed the connection unexpectedly

This probably means the server terminated abnormally

before or while processing the request.

The connection to the server was lost. Attempting reset: Failed.

```

Backend logs show:

```

[1] LOG:  client backend (PID 15420) was terminated by signal 11: Segmentation fault

[1] DETAIL:  Failed process was running: SELECT * FROM pg_get_database_ddl('db1'::regdatabase);

[1] LOG:  terminating any other active server processes

```

After applying the patch:

```

SELECT * FROM pg_get_database_ddl('db1'::regdatabase);

ERROR:  tablespace with OID 16393 does not exist

HINT:  To recover, try ALTER DATABASE ... SET TABLESPACE ... to a valid tablespace.

```

and backend logs show:

```

[56] ERROR:  tablespace with OID 16393 does not exist

[56] HINT:  To recover, try ALTER DATABASE ... SET TABLESPACE ... to a valid tablespace.

[56] STATEMENT:  SELECT * FROM pg_get_database_ddl('db1'::regdatabase);

```

All tests pass.

The only note I'd have on the code change is that there is no accompanying test. It seems like a TAP test would be reasonable, but I am quite new and will defer to whether you think that's the right call or even necessary. 

Jack