fix(tmpdir): invalid tempname() if username has slashes #19323

fix #19240

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/fileio.c

@@ -5296,6 +5296,9 @@ static void vim_mktempdir(void)
   char user[40] = { 0 };
 
   (void)os_get_username(user, sizeof(user));
+  // Usernames may contain slashes! #19240
+  memchrsub(user, '/', '_', sizeof(user));
+  memchrsub(user, '\\', '_', sizeof(user));
 
   // Make sure the umask doesn't remove the executable bit.
   // "repl" has been reported to use "0177".