treewide: Add ovs_assert to check for null pointers.

This patch adds an assortment of `ovs_assert` statements to check for
null pointers. We use assertions since it should be impossible for any
of these pointers to be NULL.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/dp-packet.c

@@ -175,6 +175,7 @@ dp_packet_new_with_headroom(size_t size, size_t headroom)
 struct dp_packet *
 dp_packet_clone(const struct dp_packet *buffer)
 {
+    ovs_assert(buffer);
     return dp_packet_clone_with_headroom(buffer, 0);
 }