Make matrix derefs report error locations correctly

Example of old behaviour:

gap> f := function()
>  S := [];
>  return S[0];
> end;
gap> f();
Error, no 1st choice method found for `[]' on 2 arguments called from
<compiled or corrupted statement>

Now correctly identifies 'S[0]' as the incorrect code.

src/vars.c

@@ -1389,6 +1389,8 @@ Obj             EvalElmList (
     /* evaluate and check the position                                     */
     pos = EVAL_EXPR( ADDR_EXPR(expr)[1] );
 
+    SET_BRK_CALL_TO(expr);     /* Note possible call for FuncWhere */
+
     if (IS_INTOBJ(pos) && (p = INT_INTOBJ( pos )) > 0)
       {