fix: assertion failure in DEBUG builds

NO_DOC=bug fix
NO_TEST=bug fix
NO_CHANGELOG=bug fix

src/lib/core/ssl_impl.c

@@ -376,6 +376,9 @@ ssl_iostream_create_supported(struct iostream *io, int fd,
 	io->fd = fd;
 	io->data = ssl;
 	io->vtab = &ssl_iostream_vtab;
+#ifndef NDEBUG
+	io->owner = NULL;
+#endif
 	return 0;
 }