Avoid 0-length memcpy to NULL with EXEC_BACKEND

memcpy(NULL, src, 0) is forbidden by POSIX, even though every
production version of libc allows it. Let's be tidy.

Per report from Thomas Munro, running UBSan with EXEC_BACKEND.
Backpatch to v17, where this code was added.

Discussion: https://www.postgresql.org/message-id/CA%2BhUKG%2Be-dV7YWBzfBZXsgovgRuX5VmvmOT%2Bv0aXiZJ-EKbXcw@mail.gmail.com

Author: hlinnaka

src/backend/postmaster/launch_backend.c

@@ -762,7 +762,8 @@ save_backend_variables(BackendParameters *param, ClientSocket *client_sock,
 	strlcpy(param->pkglib_path, pkglib_path, MAXPGPATH);
 
 	param->startup_data_len = startup_data_len;
-	memcpy(param->startup_data, startup_data, startup_data_len);
+	if (startup_data_len > 0)
+		memcpy(param->startup_data, startup_data, startup_data_len);
 
 	return true;
 }