add ctor to radio_multi, fix tearing down radio when not initialized

lib/include/srslte/radio/radio_multi.h

@@ -45,7 +45,8 @@ namespace srslte {
   class radio_multi : public radio
   {
     public: 
-      
+      radio_multi() {}
+      ~radio_multi() {}
       bool init_multi(uint32_t nof_rx_antennas, char *args = NULL, char *devname = NULL);
       bool rx_now(cf_t *buffer[SRSLTE_MAX_PORTS], uint32_t nof_samples, srslte_timestamp_t *rxd_time);
   }; 

lib/src/radio/radio.cc

@@ -81,7 +81,9 @@ bool radio::is_init() {
 
 void radio::stop() 
 {
+  if (is_initialized) {
     srslte_rf_close(&rf_device);
+  }
 }
 
 void radio::reset()