rtpsession: Avoid unnecessary copy of stats structure

rtpsession: Avoid unnecessary copy of stats structure

The code before copied GstStructure twice. The first time inside
gst_value_set_structure and the second time in g_value_array_append.
Optimized version does no copies, just transfers ownership to
GValueArray. It takes advantage of the fact that array has already
enough elements preallocated and the memory is zero initialized.

https://bugzilla.gnome.org/show_bug.cgi?id=795139