Fix edge case where perm names are not validated in custom Red decos (C…

…og-Creators#6291)

redbot/core/commands/requires.py

@@ -357,6 +357,7 @@ def decorator(func: "_CommandOrCoro") -> "_CommandOrCoro":
                 if user_perms is None:
                     func.__requires_user_perms__ = None
                 else:
+                    _validate_perms_dict(user_perms)
                     if getattr(func, "__requires_user_perms__", None) is None:
                         func.__requires_user_perms__ = discord.Permissions.none()
                     func.__requires_user_perms__.update(**user_perms)