rtpheaderextension: Don't pass a mutable output buffer reference to `write()`

The extension is only supposed to use it for potentially reading metas from it, and GstRTPBasePay is currently passing the same buffer as the one that owns the data so we currently end up with the possibility to e.g. resize the buffer which would invalidate the data.

This change prevents at least the biggest problems, but would still allow getting an immutable and mutable reference to the same data with a bit of effort.

See also #375