Improve data parsing

- add length defines for variable data header and fixed data
- add size check for fixed data
- avoid problems with memory alignment / padding in mbus structures
(improve portability)
- abort parsing if there are too many DIFE or VIFEs
- check for premature end of variable data
- check size of variable length VIF

mbus/mbus-protocol.h

@@ -192,6 +192,8 @@ typedef struct _mbus_data_variable_header {
     
 } mbus_data_variable_header;
 
+#define MBUS_DATA_VARIABLE_HEADER_LENGTH 12
+
 //
 // VARIABLE LENGTH DATA FORMAT
 //
@@ -241,6 +243,8 @@ typedef struct _mbus_data_fixed {
 
 } mbus_data_fixed;
 
+#define MBUS_DATA_FIXED_LENGTH 16
+
 //
 // ABSTRACT DATA FORMAT (error, fixed or variable length)
 //