pubsubProtocol_encodeMetadata potential NULL dereference

[Oipo]

https://github.com/apache/celix/blob/master/bundles/pubsub/pubsub_protocol/pubsub_protocol_lib/src/pubsub_wire_protocol_common.c#L196

Line refers to wrong variable, but so far all code usages either put NULL for both out arguments or non-NULL for both. Hence never triggering the NULL dereference.