Decode jump targets as offset if address is NULL

Addresses relative to the actual address of the instruction are decoded
as new offset operand, where the RIP has to be added to obtain the real
value. For backwards compatibility, the new behavior is only exposed if
the address of the instruction is specified as zero.

fadec.h

@@ -50,6 +50,7 @@ typedef enum {
     FD_OT_REG = 1,
     FD_OT_IMM = 2,
     FD_OT_MEM = 3,
+    FD_OT_OFF = 4,
 } FdOpType;
 
 typedef enum {