packets: fix potential double free vulnerabilities
packets: fix potential double free vulnerabilities

In case of a realloc failure during processing of new prefixes and router
keys the arrays where the new data is stored after initial processing are
freed twice. The first time in the processing functions rtr_store_router_key_pdu
or rtr_store_prefix_pdu, depending on the data, and during cleanup in
rtr_sync_receive_and_store_pdus which calls the earlier mentioned ones.

coverity ids: 1420788, 1420796, 1420797

rtrlib/rtrlib,rtrlib/rtrlib,rtrlib/rtrlib