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 https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/issues/375
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1399>
