ensure db is closed after block

avoid GC Warning

src/db/database.cr

@@ -22,6 +22,8 @@ module DB
     # Closes all connection to the database.
     def close
       @connection.try &.close
+      # prevent GC Warning: Finalization cycle involving discovered by mysql implementation
+      @connection = nil
     end
 
     # :nodoc:

src/db/db.cr

@@ -101,8 +101,11 @@ module DB
   # Same as `#open` but the database is yielded and closed automatically.
   def self.open(uri : URI | String, &block)
     build_database(uri).tap do |db|
-      yield db
-      db.close
+      begin
+        yield db
+      ensure
+        db.close
+      end
     end
   end