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.
7 jobs from master in 6 minutes and 36 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #92592
build autotools

00:01:33

passed #92593
build meson

00:00:41

 
  Test
passed #92595
distcheck autotools

00:02:04

passed #92597
doc-and-install meson

00:00:46

passed #92794
test autotools

00:01:22

failed #92596
allowed to fail
test meson

00:00:49

failed #92594
test autotools

01:00:00

 
Name Stage Failure
failed
test meson Test
--- command ---
BUILT_WITH_MESON='1' /builds/libnice/libnice/build/tests/nice-test-new-dribble
-------

Full log written to /builds/libnice/libnice/build/meson-logs/testlog.txt
Uploading artifacts...
build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok id=92596 responseStatus=201 Created token=sXjD-sqW
ERROR: Job failed: exit code 1