Special character in universal variable results in warning message on startup ("Unable to parse universal variable message") #10313
Description
I'm running fish 3.7.0 on Debian Linux under xterm ($TERM is xterm-256color).
Uname reports
Linux homedog 6.1.0-13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.55-1 (2023-09-29) x86_64 GNU/Linux
I tried fish with this command
sh -c 'env HOME=$(mktemp -d) XDG_CONFIG_HOME= fish'
and the effect on behavior is explained below as described in an "empty home tree."
To reproduce the problem:
$ set -U barf "tr"\xfc"mper"
$ fish
warning: Unable to parse universal variable message: 'SETUVAR barf:tr\uf6fcmper'
Welcome to fish, the friendly interactive shell
$
When the universal variable is written into fish_variables, the character \xfc is escaped into something that fish thinks is a fish_reserved_codepoint.
When I do the same thing with an empty home tree, I don't get the warning message, but the universal variable barf is incorrectly set to the empty string.