fix #505 allow forced standby mode to not assert fail

src/mesh/RadioLibInterface.cpp

@@ -236,9 +236,10 @@ void RadioLibInterface::startTransmitTimer(bool withDelay)
 void RadioLibInterface::handleTransmitInterrupt()
 {
     // DEBUG_MSG("handling lora TX interrupt\n");
-    assert(sendingPacket); // Were we sending? - FIXME, this was null coming out of light sleep due to RF95 ISR!
-
-    completeSending();
+    // This can be null if we forced the device to enter standby mode.  In that case
+    // ignore the transmit interrupt
+    if(sendingPacket)
+        completeSending();
 }
 
 void RadioLibInterface::completeSending()