manager: Fix an invalid free

The chat from .._start_chat() doesn't adds a ref
8 jobs for wip/sadiq/fix-invalid-free in 11 minutes and 39 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-pureos-gcc-byzantium #377375
librem5

00:02:26

passed build-pureos-gcc-byzantium-no-purple #377374
librem5

00:02:51

 
  Test
passed check-po #377377
librem5

00:00:17

passed test:debian-gcc #377376
librem5

00:03:21

18.9%
 
  Package
passed package-pureos-byzantium #377378
librem5

00:01:42

passed package-pureos-byzantium:arm64 #377379
librem5:arm64

00:05:07

 
  Test Package
passed autopkgtest-pureos-byzantium-package #377380
librem5

00:01:18

passed lintian-pureos-byzantium-package #377381
librem5

00:00:37