Fix what appears to be a bug in clang.

After the derive_prenonce() call, the local array cek, which had
the right value after the prior call to derive_cek(), was apparently
overwritten. So copy it before that can happen.

src/vfp.c

@@ -268,14 +268,15 @@ crypto_init(struct vfp_ctx *ctx, struct ece_crypto *crypto, uint8_t *salt,
 
 	if (vp == VFP_ERROR)
 		return (vp);
-	if (derive_cek(prk, cek, errmsg) != 0
-	    || derive_prenonce(prk, prenonce, errmsg) != 0)
+	if (derive_cek(prk, cek, errmsg) != 0)
+		return (VERR_DEC(ctx, "%s", errmsg));
+	memcpy(crypto->cek, cek, AES128_KEYLEN);
+	if (derive_prenonce(prk, prenonce, errmsg) != 0)
 		return (VERR_DEC(ctx, "%s", errmsg));
 
 	KEY_Wipe(prk);
 	crypto->prenonce_hi = vbe32dec(prenonce);
 	crypto->prenonce_lo = vbe64dec(prenonce + 4);
-	memcpy(crypto->cek, cek, AES128_KEYLEN);
 	return (VFP_OK);
 }