Explicitly check for empty ASN.1 strings in d2i_ECPrivateKey

The old code implicitly relies on the ASN.1 code returning a \0-prefixed buffer
when the buffer length is 0. Change this to verify explicitly that the ASN.1 string
has positive length.
Reviewed-by: NDr Stephen Henson <steve@openssl.org>

crypto/ec/ec_asn1.c

@@ -1195,14 +1195,20 @@ EC_KEY *d2i_ECPrivateKey(EC_KEY **a, const unsigned char **in, long len)
 	if (priv_key->publicKey)
 		{
 		const unsigned char *pub_oct;
-		size_t pub_oct_len;
+		int pub_oct_len;
 
 		pub_oct     = M_ASN1_STRING_data(priv_key->publicKey);
 		pub_oct_len = M_ASN1_STRING_length(priv_key->publicKey);
-		/* save the point conversion form */
+		/* The first byte - point conversion form - must be present. */
+                if (pub_oct_len <= 0)
+			{
+			ECerr(EC_F_D2I_ECPRIVATEKEY, EC_R_BUFFER_TOO_SMALL);
+			goto err;
+			}
+		/* Save the point conversion form. */
 		ret->conv_form = (point_conversion_form_t)(pub_oct[0] & ~0x01);
 		if (!EC_POINT_oct2point(ret->group, ret->pub_key,
-			pub_oct, pub_oct_len, NULL))
+					pub_oct, (size_t)(pub_oct_len), NULL))
 			{
 			ECerr(EC_F_D2I_ECPRIVATEKEY, ERR_R_EC_LIB);
 			goto err;