fix false negative for CVE-2014-7169 (taviso bug)

I fixed a test after spotting this weird result on SUSE Linux Enterprise Server 11:

# ./bashcheck 
Testing /bin/bash ...
GNU bash, version 3.2.51(1)-release (x86_64-suse-linux-gnu)
Vulnerable to CVE-2014-6271 (original shellshock)
Not vulnerable to CVE-2014-7169 (taviso bug)
Vulnerable to CVE-2014-7186 (redir_stack bug)
Test for CVE-2014-7187 not reliable without address sanitizer
./bashcheck: line 42: 10469 Segmentation fault      $bash -c "f(){ x(){ _;};x(){ _;}<<a;}"
Vulnerable to CVE-2014-6277 (lcamtuf bug #1) [no patch]
Variable function parser still active, maybe vulnerable to unknown parser bugs