Assign Default Value expansion inserts ASCII DEL for empty 'word'

[Martin Castillo]

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
-Wno-parentheses -Wno-format-security
uname output: Linux arctic 5.5.6-artix1-1 #1 SMP PREEMPT Tue, 25 Feb
2020 14:57:48 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 16
Release Status: release

Description:
Out of nowhere, bash inserts an ascii DEL in the first expansion. That
is very unexpected and shouldn't be there.

Repeat-By:
$ unset foo bar
$ echo "${foo:=}/baz:${foo}/bat"|xxd
00000000: 7f2f 6261 7a3a 2f62 6174 0a              ./baz:/bat.
$ echo "${foo:=$bar}/baz:${foo}/bat"|xxd
00000000: 7f2f 6261 7a3a 2f62 6174 0a              ./baz:/bat.