Replace magic value with defines
Add -Wfree-nonheap-object to cmake debug config
Fix: free non-heap object

rtr_store_prefix_pdu and rtr_store_router_key_pdu
On realloc error both functions would have freed a non-heap object.
Merge branch 'rfc6810-bis' of github.com:rtrlib/rtrlib into rfc6810-bis