crash when using backquoted commands in $PS4

[Matias Atria]

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DSHELL -DHAVE_CONFIG_H  -D_FILE_OFFSET_BITS=64  -I.  -I. 
-I./lib -I/usr/local/include -O2
uname output: Linux silvio 2.4.14 #2 Thu Dec 20 12:14:39 EST 2001 i686 unknown
Machine Type: i686-pc-linux-gnu

Bash Version: 2.03
Patch Level: 0
Release Status: release

Description:
When setting PS4 to a backquoted command, bash keeps forking subshells
until it crashes.

$ PS4='`ls`' sh -x -c echo
Segmentation violation
$ PS4='`ls`' sh -x -c echo
[suspended after a couple of seconds with ^Z]
$ ps aux | grep 'sh -x -c echo' | wc -l
1164

Repeat-By:
PS4='`ls`' sh -x -c echo