[ruby/openssl] pkey/ec: check existence of public key component before exporting

i2d_PUBKEY_bio() against an EC_KEY without the public key component
trggers a null dereference.

This is a regression introduced by commit ruby/openssl@56f0d34d63fb ("pkey:
refactor #export/#to_pem and #to_der", 2017-06-14).

Fixes ruby/openssl#527 (comment)
Fixes ruby/openssl#369 (comment)

ruby/openssl@f6ee0fa4de

Author: rhenium

ext/openssl/ossl_pkey_ec.c

@@ -414,6 +414,8 @@ ossl_ec_key_export(int argc, VALUE *argv, VALUE self)
     EC_KEY *ec;
 
     GetEC(self, ec);
+    if (EC_KEY_get0_public_key(ec) == NULL)
+        ossl_raise(eECError, "can't export - no public key set");
     if (EC_KEY_get0_private_key(ec))
         return ossl_pkey_export_traditional(argc, argv, self, 0);
     else
@@ -432,6 +434,8 @@ ossl_ec_key_to_der(VALUE self)
     EC_KEY *ec;
 
     GetEC(self, ec);
+    if (EC_KEY_get0_public_key(ec) == NULL)
+        ossl_raise(eECError, "can't export - no public key set");
     if (EC_KEY_get0_private_key(ec))
         return ossl_pkey_export_traditional(0, NULL, self, 1);
     else

test/openssl/test_pkey_ec.rb

@@ -61,8 +61,10 @@ def test_generate
   def test_generate_key
     ec = OpenSSL::PKey::EC.new("prime256v1")
     assert_equal false, ec.private?
+    assert_raise(OpenSSL::PKey::ECError) { ec.to_der }
     ec.generate_key!
     assert_equal true, ec.private?
+    assert_nothing_raised { ec.to_der }
   end if !openssl?(3, 0, 0)
 
   def test_marshal