Add marker for invalid frame_index

So far we used 0 but this is not valid int he Eiger case
where the frame_index gets set to 0 in each acquisition.

core-buffer/include/formats.hpp

@@ -1,6 +1,8 @@
 #ifndef SF_DAQ_BUFFER_FORMATS_HPP
 #define SF_DAQ_BUFFER_FORMATS_HPP
 
+#define INVALID_FRAME_INDEX UINT64_C(-1)
+
 #pragma pack(push)
 #pragma pack(1)
 struct ModuleFrame {