ns_spki: fix exception class in OpenSSL::Netscape::SPKI#to_der

It should raise OpenSSL::Netscape::SPKIError instead of
OpenSSL::X509::CertificateError.

No test cases covered this because it only occurs in exceptional
cases, such as memory allocation failure.

ext/openssl/ossl_ns_spki.c

@@ -115,11 +115,11 @@ ossl_spki_to_der(VALUE self)
 
     GetSPKI(self, spki);
     if ((len = i2d_NETSCAPE_SPKI(spki, NULL)) <= 0)
-        ossl_raise(eX509CertError, NULL);
+        ossl_raise(eSPKIError, "i2d_NETSCAPE_SPKI");
     str = rb_str_new(0, len);
     p = (unsigned char *)RSTRING_PTR(str);
     if (i2d_NETSCAPE_SPKI(spki, &p) <= 0)
-        ossl_raise(eX509CertError, NULL);
+        ossl_raise(eSPKIError, "i2d_NETSCAPE_SPKI");
     ossl_str_adjust(str, p);
 
     return str;