mongo-sync: Fix a memory leak

When _mongo_sync_packet_send() is called, all callers expect the passed
packet to get free()'d in all cases. This did not happen if we had
force_master set, and _mongo_cmd_ensure_conn() failed. This patch fixes
this path too, and fixes #26.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>