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/shash.c

@@ -265,7 +265,7 @@ void *
 shash_find_and_delete_assert(struct shash *sh, const char *name)
 {
     void *data = shash_find_and_delete(sh, name);
-    ovs_assert(data != NULL);
+    ovs_assert(data);
     return data;
 }