Fix bytecode for block+mfs>0 in static model

Variable indices would be incorrect in the evaluated Jacobian if recursive
variables were present. This would lead to incorrect results and/or crashes in
bytecode MEX. This bug has been exposed by commit
f45a99fc68, which actually enabled mfs>0 for
static models.

src/StaticModel.hh

@@ -87,7 +87,8 @@ private:
   int
   getBlockJacobianEndoCol([[maybe_unused]] int blk, int var, [[maybe_unused]] int lag) const override
   {
-    return var;
+    assert(var >= blocks[blk].getRecursiveSize());
+    return var - blocks[blk].getRecursiveSize();
   }
 
   // Write the block structure of the model in the driver file