Bytecode: fix FSTPG3_::get_lag()

It would return an incorrect result. This had no impact except for debugging
information.

src/Bytecode.hh

@@ -446,7 +446,7 @@ public:
   int
   get_lag()
   {
-    return arg2;
+    return arg3;
   };
   int
   get_col_pos()