fix protocol_splitter: increment i properly in scan_for_packets

This also guarantees that i is increased in every loop iteration.
Before it was possible to enter a busy loop.

src/drivers/protocol_splitter/protocol_splitter.cpp

@@ -548,7 +548,7 @@ void DevCommon::scan_for_packets()
 
 		perf_set_count(header_bytes_received_count, _read_buffer->header_bytes_received);
 
-		i += payload_len;
+		i += Sp2HeaderSize + payload_len;
 
 	}
 }