Add safeguards against signalling peer-reflexive candidates.

BUG=4208
R=juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/50799004

Cr-Commit-Position: refs/heads/master@{#9104}
diff --git a/talk/app/webrtc/webrtcsdp.cc b/talk/app/webrtc/webrtcsdp.cc
index 72a72bf..a93977d 100644
--- a/talk/app/webrtc/webrtcsdp.cc
+++ b/talk/app/webrtc/webrtcsdp.cc
@@ -1770,6 +1770,8 @@
       type = kCandidateRelay;
     } else {
       ASSERT(false);
+      // Never write out candidates if we don't know the type.
+      continue;
     }
 
     InitAttrLine(kAttributeCandidate, &os);