Transceiver: explicitly init m{Rx,Tx}LowerLoopThread

Coverity warns us that a non-static class members:

  - mRxLowerLoopThread, and
  - mTxLowerLoopThread,

are not initialized in this constructor nor in any functions that
it calls.  I don't think it's critical, because we do initialize
them in Transceiver::start(), but let's make them nullptr.

Change-Id: If9e06aa7965f17383ab6599c15945e8ce2703bbf
Fixes: CID#214952

Transceiver52M/Transceiver.cpp

@@ -136,6 +136,7 @@ Transceiver::Transceiver(const struct trx_cfg *cfg,
                          GSM::Time wTransmitLatency,
                          RadioInterface *wRadioInterface)
   : cfg(cfg), mClockSocket(-1),
+    mRxLowerLoopThread(nullptr), mTxLowerLoopThread(nullptr),
     mTransmitLatency(wTransmitLatency), mRadioInterface(wRadioInterface),
     mChans(cfg->num_chans), mOn(false), mForceClockInterface(false),
     mTxFreq(0.0), mRxFreq(0.0), mTSC(0), mMaxExpectedDelayAB(0),