Allow ScopedDataChannel::Close() to be called  multiple times

No reason not to, and some code path was managing to hit it.

Change-Id: I8e3aa505474df4de0b9e90e728e51114dd5d44bf
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/aos/network/rawrtc.cc b/aos/network/rawrtc.cc
index 98f2448..c195a4b 100644
--- a/aos/network/rawrtc.cc
+++ b/aos/network/rawrtc.cc
@@ -148,8 +148,9 @@
 
 void ScopedDataChannel::Close() {
   CHECK(opened_);
-  CHECK(!closed_);
-  CHECK_RAWRTC(rawrtc_data_channel_close(data_channel_));
+  if (!closed_) {
+    CHECK_RAWRTC(rawrtc_data_channel_close(data_channel_));
+  }
 }
 
 void ScopedDataChannel::Send(const ::flatbuffers::DetachedBuffer &buffer) {