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.

(Imported from upstream's 7f7c05ca638c3cc6d261961fae439cd91e3c1d27)

Change-Id: Icc6c44b874bdcb02374016a36d209830d6162a8a

crypto/ec/ec_asn1.c

diff --git a/crypto/ec/ec_asn1.c b/crypto/ec/ec_asn1.c
index add2f53..e7d96e1 100644
--- a/crypto/ec/ec_asn1.c
+++ b/crypto/ec/ec_asn1.c
@@ -352,11 +352,16 @@
 
   if (priv_key->publicKey) {
     const uint8_t *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 (the point conversion form) must be present. */
+    if (pub_oct_len <= 0) {
+      OPENSSL_PUT_ERROR(EC, 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)) {