Patch bash42-005 causes SIGSEGV

[Evangelos Foutras]

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-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' 
-DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -march=x86-64 
-mtune=generic -O2 -pipe 
-DDEFAULT_PATH_VALUE='/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin'
 -DSTANDARD_UTILS_PATH='/usr/bin:/bin:/usr/sbin:/sbin' 
-DSYS_BASHRC='/etc/bash.bashrc' -DSYS_BASH_LOGOUT='/etc/bash.bash_logout'
uname output: Linux foutbox 2.6.37-ARCH #1 SMP PREEMPT Fri Feb 18 18:32:16 CET 
2011 x86_64 AMD Phenom(tm) II X6 1090T Processor AuthenticAMD GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.2
Patch Level: 5
Release Status: release

Description:
        It appears that the chkexport function introduced in this patch level
        is prone to a segmentation fault.

Repeat-By:
        A simple `bash -c 'unset TZ'` is enough to trigger the segmentation
        fault on my system.

Fix:
        Maybe check the value returned by find_variable in chkexport to ensure
        it's not NULL? I noticed this is done in other places where
        find_variable is used, so I think it makes sense to do it here too.