EC: Reverse the default asn1_flag in a new EC_GROUP

The default was OPENSSL_EC_NAMED_CURVE, but that's not true until a
curve name has been set, so we change the initial value to
OPENSSL_EC_EXPLICIT_CURVE and let EC_GROUP_set_curve_name() change it
to OPENSSL_EC_NAMED_CURVE.

Submitted by Matt Caswell
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13973)

crypto/ec/ec_lib.c

@@ -63,7 +63,7 @@ EC_GROUP *ec_group_new_ex(OSSL_LIB_CTX *libctx, const char *propq,
         if (ret->cofactor == NULL)
             goto err;
     }
-    ret->asn1_flag = OPENSSL_EC_NAMED_CURVE;
+    ret->asn1_flag = OPENSSL_EC_EXPLICIT_CURVE;
     ret->asn1_form = POINT_CONVERSION_UNCOMPRESSED;
     if (!meth->group_init(ret))
         goto err;
@@ -481,6 +481,10 @@ const BIGNUM *EC_GROUP_get0_cofactor(const EC_GROUP *group)
 void EC_GROUP_set_curve_name(EC_GROUP *group, int nid)
 {
     group->curve_name = nid;
+    group->asn1_flag =
+        (nid != NID_undef)
+        ? OPENSSL_EC_NAMED_CURVE
+        : OPENSSL_EC_EXPLICIT_CURVE;
 }
 
 int EC_GROUP_get_curve_name(const EC_GROUP *group)