fix multline log messages

[bashtanov]

currently they are lost

[bashtanov]

@petere when a multiline message is to be logged an empty one appears instead. This is to fix it.

I found that when trying to apply https://github.com/pgbouncer/pgbouncer/blob/master/test/asynctest.c to pgbouncer/pgbouncer#420.

I would appreciate if you could have a look.

[petere]

Where are multiline log messages used?

[bashtanov]

here the reason may come from libpq and can be multiline, e.g. like this:

2019-10-04 17:14:58.925 BST [5170] DEBUG disconnect because: PQconnectStart: could not connect to server: Cannot assign requested address
 		Is the server running on host "127.0.0.1" and accepting
 		TCP/IP connections on port 7000?

Not sure if pgbouncer itself can attempt emitting multiline messages.