lib: fix potential segfault in _keyslot_repair

The value of vk may be NULL in _keyslot_repair. It will
be dereferenced in LUKS_generate_phdr. Check it to avoid
segfault.

Signed-off-by: Lixiaokeng <lixiaokeng@huawei.com>
Signed-off-by: Linfeilong <linfeilong@huawei.com>

lib/luks1/keymanage.c

@@ -386,6 +386,8 @@ static int _keyslot_repair(struct luks_phdr *phdr, struct crypt_device *ctx)
 		return -EINVAL;
 
 	vk = crypt_alloc_volume_key(phdr->keyBytes, NULL);
+	if (!vk)
+		return -ENOMEM;
 
 	log_verbose(ctx, _("Repairing keyslots."));