memory leak when signer failed

The signature structure does not free memory when the signer failed because of network abnormal.
when signer fails, the old rrsig record will be stored in the zone->ixf->[0] in memory, but it does not fully released when write files. I think we could add ldns_rr_free(signature) on line 2771, 2784 and 2791 of libhsm.c to resolve it.