component: Fix use-after-free

conn_check_prune_socket() was trying to access deleted NiceCandidate
instance. Prune the socket before removing the candidate.

Closes #73
7 jobs for heap-use-after-free in 8 minutes and 33 seconds (queued for 55 minutes and 34 seconds)
Status Job ID Name Coverage
  Build
passed #164893
build autotools

00:01:14

passed #164894
build meson

00:00:30

 
  Test
passed #164896
distcheck autotools

00:02:53

passed #164898
doc-and-install meson

00:01:02

passed #165095
test autotools

00:01:55

failed #164897
allowed to fail
test meson

00:00:57

failed #164895
test autotools

00:02:06

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

Full log written to /builds/xhaakon/libnice/build/meson-logs/testlog.txt
Uploading artifacts...
build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok
id=164897 responseStatus=201 Created token=WiEyyZxJ
ERROR: Job failed: exit code 1