component_io_cb: Avoid potential deadlock

Don't unref NiceAgent while holding agent mutex. If it's the last
reference, nice_agent_dispose() will try to acquire the mutex again and
deadlock.
6 jobs for <span class="ref-name">fix-unref</span> in 2 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #92322
build autotools

00:00:53

passed #92323
build meson

00:00:25

 
  Test
passed #92325
distcheck autotools

00:02:00

passed #92327
doc-and-install meson

00:00:39

passed #92324
test autotools

00:01:23

passed #92326
allowed to fail
test meson

00:00:41