db: schema_tables: capture reference to temporary value by value

`clustering_key_columns()` returns a range view, and `front()` returns
the reference to its first element. so we cannot assume the availability
of this reference after the expression is evaluated. to address this
issue, let's capture the reference by value.

this also silences warning from GCC-13:

```
/home/kefu/dev/scylladb/db/schema_tables.cc:3654:30: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 3654 |     const column_definition& first_view_ck = v->clustering_key_columns().front();
      |                              ^~~~~~~~~~~~~
/home/kefu/dev/scylladb/db/schema_tables.cc:3654:79: note: the temporary was destroyed at the end of the full expression ‘(& v)->view_ptr::operator->()->schema::clustering_key_columns().boost::iterator_range<__gnu_cxx::__normal_iterator<const column_definition*, std::vector<column_definition> > >::<anonymous>.boost::iterator_range_detail::iterator_range_base<__gnu_cxx::__normal_iterator<const column_definition*, std::vector<column_definition> >, boost::iterators::random_access_traversal_tag>::<anonymous>.boost::iterator_range_detail::iterator_range_base<__gnu_cxx::__normal_iterator<const column_definition*, std::vector<column_definition> >, boost::iterators::bidirectional_traversal_tag>::<anonymous>.boost::iterator_range_detail::iterator_range_base<__gnu_cxx::__normal_iterator<const column_definition*, std::vector<column_definition> >, boost::iterators::incrementable_traversal_tag>::front()’
 3654 |     const column_definition& first_view_ck = v->clustering_key_columns().front();
      |                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
```

Fixes scylladb#13720
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

db/schema_tables.cc

@@ -3651,7 +3651,7 @@ view_ptr maybe_fix_legacy_secondary_index_mv_schema(replica::database& db, const
     if (v->clustering_key_size() == 0) {
         return view_ptr(nullptr);
     }
-    const column_definition& first_view_ck = v->clustering_key_columns().front();
+    const column_definition first_view_ck = v->clustering_key_columns().front();
     if (first_view_ck.is_computed()) {
         return view_ptr(nullptr);
     }