Writing to /dev/stderr overwrites xtrace data

[jono . chadwell]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -Wdate-time 
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat 
-Werror=format-security -Wall
uname output: Linux birch 4.15.0-65-generic #74~16.04.1-Ubuntu SMP Wed Sep 18 
09:51:44 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 48
Release Status: release

Description:
    I enabled xtrace to try and debug a bash script running on a validation
    server. The script stored its stdout and stderr to a log file. I expected
    to see trace data in the log file, but instead found a single log statement,
    a bunch of NULL bytes, and the line '+ exit 1'. 

    After some playing around with it I found that writing to the file
    /dev/stderr was responsible for deleting the xtrace data.

Repeat-By:


```
echo "#!/usr/bin/env bash" > script.sh
echo "set -o xtrace" >> script.sh
echo "echo to_stderr > /dev/stderr" >> script.sh
chmod 755 script.sh
./script.sh 2> stderr_output
```

    After running these commands I expect to see find xtrace output in
    stderr_output. Instead I only see 'to_stderr\n'.


If this behavior is unavoidable due to how linux file IO, then I recommend at
least adding the documentation as a known gotcha.

Thanks,

Jono Chadwell