channel: don't call memcpy with NULL dest
channel: don't call memcpy with NULL dest

If ncaps == 0, the array will be NULL after set_size().

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@redhat.com>
Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

flexVDI/spice-gtk,flexVDI/spice-gtk