Do not try to cleanup statements (MagicStack#981)

This supports a case where we prepare an unnamed statement to inspect the return types.
The statement should not be cleaned up afterwards because it is automatically cleaned up by Postgres

Author: fvannee

asyncpg/connection.py

@@ -1416,6 +1416,7 @@ def _mark_stmts_as_closed(self):
     def _maybe_gc_stmt(self, stmt):
         if (
             stmt.refs == 0
+            and stmt.name
             and not self._stmt_cache.has(
                 (stmt.query, stmt.record_class, stmt.ignore_custom_codec)
             )