Transceiver52M: Check time slot validity of incoming bursts

In errant cases, GSM core may send bursts with invalid slot values,
which is allowed by the GSM::Time object. If we find a burst like this
coming into the transceiver, then drop it immediately.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
diff --git a/Transceiver52M/Transceiver.cpp b/Transceiver52M/Transceiver.cpp
index eae4e06..33d22ad 100644
--- a/Transceiver52M/Transceiver.cpp
+++ b/Transceiver52M/Transceiver.cpp
@@ -176,6 +176,11 @@
     return;
   }
 
+  if (wTime.TN() > 7) {
+    LOG(ALERT) << "Received burst with invalid slot " << wTime.TN();
+    return;
+  }
+
   burst = modulateBurst(bits, 8 + (wTime.TN() % 4 == 0), mSPSTx);
   scaleVector(*burst, txFullScale * pow(10, -RSSI / 10));