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.

diff --git a/gst/rtpmanager/gstrtprtxsend.c b/gst/rtpmanager/gstrtprtxsend.c
index 4329d84..58892f3 100644
--- a/gst/rtpmanager/gstrtprtxsend.c
+++ b/gst/rtpmanager/gstrtprtxsend.c
@@ -400,7 +400,10 @@
   /* copy extension if any */
   if (rtp.size[1]) {
-    mem = gst_memory_copy (rtp.map[1].memory, 0, rtp.size[1]);
+    mem = gst_allocator_alloc (NULL, rtp.size[1], NULL);
+    gst_memory_map (mem, &map, GST_MAP_WRITE);
+    memcpy (map.data, rtp.data[1], rtp.size[1]);
+    gst_memory_unmap (mem, &map);
     gst_buffer_append_memory (new_buffer, mem);