snap: fix race condition

With the SNAP out-of-band SCTP connect (WebRTC-Sctp-Snap), the dcSCTP
socket is created in DcSctpTransport::Start() but only connected later
in MaybeConnectSocket(), once the DTLS transport becomes writable. The
DTLS transport can decrypt and deliver an inbound packet as soon as its
handshake completes, which precedes writability (set_writable() is
deferred until ICE is writable). A packet arriving in that window is
delivered to the still-kClosed socket, which rejects it as having an
invalid verification tag.

End-to-end this is masked by SCTP retransmission, so it only surfaces as
an 'invalid verification tag' log line and a one-RTO delay opening the
data channel. The dcsctp_transport unit test observes the drop directly
via the mock socket.

Bug: webrtc:526609852
Change-Id: Iff17f2deb766ae465d68728d613e8c17d046263e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/484720
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Denise Tell <dct@google.com>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#48061}