Transceiver52M: UHD: Check running status before stopping stream

On startup errors we get a segfault if we stop and shutdown. This
is because we try to send a stop stream command to the device before
it has been created. Setup a check for running status before
attempting to stop the physical device.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

Transceiver52M/UHDDevice.cpp

@@ -756,7 +756,10 @@ bool uhd_device::start()
 
 bool uhd_device::stop()
 {
-	uhd::stream_cmd_t stream_cmd = 
+	if (!started)
+		return false;
+
+	uhd::stream_cmd_t stream_cmd =
 		uhd::stream_cmd_t::STREAM_MODE_STOP_CONTINUOUS;
 
 	usrp_dev->issue_stream_cmd(stream_cmd);