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