lib/util: clang: Fix dereference of a null pointer warning

lib/util: clang: Fix dereference of a null pointer warning

Fixes:

lib/util/rbtree.c:170:8: warning: Access to field 'rb_parent_color' results in a dereference of a null pointer (loaded from variable 'other') <--[clang]

We could avoid accessing the NULL pointer but previously the code would
have crashed here. Given this is a rbtree probably better to preserve the
fatal nature of encountering a NULL pointer here while satisfying the static
checker.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Gary Lockyer gary@catalyst.net.nz