Fix two test failures with functions marked nodiscard.

With gcc-15, the dereferencing or the + operator of iterators are marked
nodiscard. This makes two tests fail, because the warning leads to a
non-zero diff.

cling/dict/runoperators.C

@@ -23,6 +23,6 @@ void runoperators() {
    std::vector<myiterator> v;
    std::vector<myiterator>::iterator iter;
    for( iter = v.begin(); iter != v.end(); ++iter) {
-      iter + 1;
+      static_cast<void>(= iter + 1);
    }
 }

cling/stl/default/VectorSort.C

@@ -53,7 +53,7 @@ int VectorSort() {
    // ensure that we can iterate, and that the precedence is correct,
    // and that the op++ doesn't operator on the elements:
    ++beg++;
-   *(++beg)++;
+   static_cast<void>(*(++beg)++);
    ++(*(++beg));
    beg = ++beg++;
    beg = ++beg++;