pipeline: Declare pipeline dead only once

The pipeline was being declared dead first when an error was reported on
the bus, and second when the async-done was not being received.

Fixes #2232
4 jobs from master in 78 minutes and 38 seconds
Status Job ID Name Coverage
  Docker Image
passed #92176
pitivi:1.0

00:56:25

passed #92175
pitivi:master

00:57:14

 
  Build
passed #92177
build

00:17:57

 
  Test Results
passed #92178
test-results

00:03:26