### Description
I'm noticing an issue with certain RPC after upgrading to 5.6. I'm using "db_postgres".
If I run ul.flush like this: ``` kamcmd ul.flush # no output ```
None of the in-memory registrations are flushed to the DB and in the log I see a bunch of lines like this: ``` May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: <core> [db.c:481]: db_use_table(): invalid connection parameter May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: usrloc [ucontact.c:1142]: db_update_ucontact_ruid(): sql use_table failed May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: usrloc [urecord.c:433]: wb_timer(): updating contact in db failed (aor: fake-1@aor.com) May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: <core> [db.c:481]: db_use_table(): invalid connection parameter May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: usrloc [ucontact.c:1142]: db_update_ucontact_ruid(): sql use_table failed May 12 20:39:16 <dummy-hostname> /usr/sbin/kamailio[9006]: ERROR: usrloc [urecord.c:433]: wb_timer(): updating contact in db failed (aor: fake-2@aor.com) ```
I see the same thing with ul.db_users: ``` kamcmd ul.db_users location # returns the following output error: 500 - Failed to use table
# in the log ERROR: <core> [db.c:481]: db_use_table(): invalid connection parameter
```
### Troubleshooting
#### Reproduction
Seems to happen consistently on 5.6, using psql as the db engine. I tested on 5.4 and it's not happening.
It's easier to test with ```kamcmd ul.db_users location``` because you need registrations that haven't been flushed to the DB for the error to manifest.
* **Operating System**: ``` Distributor ID: Debian Description: Debian GNU/Linux 10 (buster) Release: 10 Codename: buster
Kernel: 4.19.0-21-cloud-amd64 ```