Add explicit fall through comments. (#1018)

src/core/thread/lowpan.cpp

@@ -411,6 +411,8 @@ int Lowpan::Compress(Message &aMessage, const Mac::Address &aMacSource, const Ma
 
             cur += Compress(aMessage, aMacSource, aMacDest, cur);
 
+        // fall through
+
         default:
             ExitNow();
         }

src/core/thread/mle_router.cpp

@@ -1724,6 +1724,8 @@ void MleRouter::HandleStateUpdateTimer(void)
             ExitNow();
         }
 
+    // fall through
+
     case kDeviceStateRouter:
         // verify path to leader
         otLogDebgMle("network id timeout = %d", GetLeaderAge());