openssl: Return correct number of plugin features if ECDH groups are omitted

Fixes: 46a6b06282 ("openssl: Only announce ECDH groups actually supported by OpenSSL")

src/libstrongswan/plugins/openssl/openssl_plugin.c

@@ -881,7 +881,7 @@ METHOD(plugin_t, get_features, int,
 #endif
 	}
 	*features = f;
-	return countof(f);
+	return count;
 }
 
 METHOD(plugin_t, destroy, void,