Fixed null pointer dereference in _signet_get_msg_sign_keys
Fixed null pointer dereference in _signet_get_msg_sign_keys

Found by Coverity Scan, CID 27653

greyg00s/libdime,greyg00s/libdime,lavabit/libdime,lavabit/libdime,lavabit/libdime,lavabit/libdime