rtxsend: fix wrong memory layout assumption

The code responsible for creating retransmitted buffers
assumed the stored buffer had been created with
rtp_buffer_new_allocate when copying the extension data,
which isn't necessarily the case, for example when
the rtp buffers come from a udpsrc.

https://bugzilla.gnome.org/show_bug.cgi?id=794958
1 file changed