tab completing `/ cause weird errors messages.

[Emanuele Torre]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=x86-64 -mtune=generic -O2 -pipe -fno-plt
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin'
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc'
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
-DNON_INTERACTIVE_LOGIN_SHELLS
uname output: Linux t420 5.15.48-1-lts #1 SMP Thu, 16 Jun 2022
13:32:35 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 16
Release Status: release

Description:
    Tab completing `/ (backtick-slash) causes bash to print lots of
    weird error messages.

Repeat-By:
    Repoduce interactively:

     bash-5.1$ `/<press-tab-twice>
     binbash: bad substitution: no closing "`" in `/bin
              devbash: bad substitution: no closing "`" in `/dev
              homebash: bad substitution: no closing "`" in `/home
             lib64bash: bad substitution: no closing "`" in `/lib64
            mntbash: bad substitution: no closing "`" in `/mnt
              procbash: bad substitution: no closing "`" in `/proc
             runbash: bad substitution: no closing "`" in `/run
              srvbash: bad substitution: no closing "`" in `/srv
              tmpbash: bad substitution: no closing "`" in `/tmp
              varbash: bad substitution: no closing "`" in `/var

     bootbash: bad substitution: no closing "`" in `/boot
             etcbash: bad substitution: no closing "`" in `/etc
              libbash: bad substitution: no closing "`" in `/lib
              lost+foundbash: bad substitution: no closing "`" in `/lost+found
       optbash: bad substitution: no closing "`" in `/opt
              rootbash: bad substitution: no closing "`" in `/root
             sbinbash: bad substitution: no closing "`" in `/sbin
             sysbash: bad substitution: no closing "`" in `/sys
              usrbash: bad substitution: no closing "`" in `/usr

     bash-5.1$ `/

    Reproduce with a command line:

     bash-5.1$ printf '`/\t\t' | PS1='$ ' bash --norc --noprofile -i |& sed -n l
     $ `/\a$
     binbash: bad substitution: no closing "`" in `/bin$
              devbash: bad substitution: no closing "`" in `/dev$
              homebash: bad substitution: no closing "`" in `/home$
             lib64bash: bad substitution: no closing "`" in `/lib64$
            mntbash: bad substitution: no closing "`" in `/mnt$
              procbash: bad substitution: no closing "`" in `/proc$
             runbash: bad substitution: no closing "`" in `/run$
              srvbash: bad substitution: no closing "`" in `/srv$
              tmpbash: bad substitution: no closing "`" in `/tmp$
              varbash: bad substitution: no closing "`" in `/var$
              $
     bootbash: bad substitution: no closing "`" in `/boot$
             etcbash: bad substitution: no closing "`" in `/etc$
              libbash: bad substitution: no closing "`" in `/lib$
              lost+foundbash: bad substitution: no closing "`" in `/lost+f\

       optbash: bad substitution: no closing "`" in `/opt$
              rootbash: bad substitution: no closing "`" in `/root$
             sbinbash: bad substitution: no closing "`" in `/sbin$
             sysbash: bad substitution: no closing "`" in `/sys$
              usrbash: bad substitution: no closing "`" in `/usr$
              $
     $ `/$
     > bash: unexpected EOF while looking for matching ``'$
     bash: syntax error: unexpected end of file$
     $ exit$

    I was able to reproduce this bug in bash5 and bash4. but not in
    bash3, in which it behaves correctly (suggesting the files in / as
    completion options).