crypto: fix uninitialized variable coverity warning

For some encrypt operations DPP passes no AD iovecs (both are
NULL/0). But since the iovec itself is on the stack 'ad' is a
valid pointer from within aes_siv_encrypt. This causes memcpy
to be called which coverity complains about. Since the copy
length is zero it was effectively a no-op, but check num_ad to
prevent the call.

src/crypto.c

@@ -331,7 +331,7 @@ bool aes_siv_encrypt(const void *key, size_t key_len, const void *in,
 	struct iovec iov[num_ad + 1];
 	uint8_t v[16];
 
-	if (ad)
+	if (ad && num_ad)
 		memcpy(iov, ad, sizeof(struct iovec) * num_ad);
 
 	iov[num_ad].iov_base = (void *)in;