metadata: try to upgrade Weak before returning a pointer

Weak::as_ptr() can return an invalid pointer. It can be even dangling (non-null).

It's safer to try to upgrade to an Arc. If upgrade was successful,
make use of RefFFI API to return a valid pointer. Otherwise, return
non-dangling null pointer.

scylla-rust-wrapper/src/metadata.rs

@@ -433,7 +433,11 @@ pub unsafe extern "C" fn cass_materialized_view_meta_base_table(
     view_meta: *const CassMaterializedViewMeta,
 ) -> *const CassTableMeta {
     let view_meta = RefFFI::as_ref(view_meta);
-    view_meta.base_table.as_ptr()
+
+    match view_meta.base_table.upgrade() {
+        Some(arc) => RefFFI::as_ptr(&arc),
+        None => std::ptr::null(),
+    }
 }
 
 #[no_mangle]