Handle baseband::dma::wait_for_rx_buffer() returning empty buffer.

Was technically OK before, because sample count was zero. But seems silly (and vaguely dangerous) to call all that code with a nullptr.

firmware/baseband/baseband_thread.cpp

@@ -98,6 +98,7 @@ void BasebandThread::run() {
 	while(true) {
 		// TODO: Place correct sampling rate into buffer returned here:
 		const auto buffer_tmp = baseband::dma::wait_for_rx_buffer();
+		if( buffer_tmp ) {
 			buffer_c8_t buffer {
 				buffer_tmp.p, buffer_tmp.count, baseband_configuration.sampling_rate
 			};
@@ -113,6 +114,7 @@ void BasebandThread::run() {
 				}
 			);
 		}
+	}
 
 	delete baseband_buffer;
 }

firmware/common/buffer.hpp

@@ -92,6 +92,10 @@ struct buffer_t {
 		timestamp { timestamp }
 	{
 	}
+
+	operator bool() const {
+		return (p != nullptr);
+	}
 };
 
 #endif/*__BUFFER_H__*/