fixed keyring-related memory leaks

[dismantl]

to test:

1. build serval-crypto against this version of serval-dna
2. make sure servald is running, and has only one key in its keyring
3. With Valgrind, run either the signing function (valgrind --leak-check=yes --show-reachable=yes ./serval-crypto --sign -i <SID> -m <MSG>) or verify function (valgrind --leak-check=yes --show-reachable=yes ./ serval-crypto --verify -i <SID> -m <MSG> -s <SIG>).
4. Valgrind should report 0 bytes {definitely,indirectly,possibly} lost (though there will be some blocks still reachable).