webrtc: fix some data channel related leaks
Partial fix for #1383 (closed)
commit 2b4de9fd
webrtc/datachannel: clear the error after use
Fixes a memory leak
commit f2208d3c
webrtc/datachannel: free previous protocol/label fields
Fixes a memory leak
commit eca5aeac
tests/webrtc: unref GBytes after use
commit d24620af
sctpdec: unref after retrieving the static pad template