19611: sys/net/rpl: fix possible NULL dereference r=benpicco a=maribu
### Contribution description
As the title says
19640: core/thread: drop unused thread_arch_t r=benpicco a=maribu
### Contribution description
No architecture makes use of thread_arch_t anymore, so let's drop it.
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>