pigz: fix one failure of command "unpigz -l"

Refer [1], "unpigz -l" failed with error:
$ ./unpigz -l test.txt.gz
compressed original reduced name
228799 209715200 99.9% test.txt
unpigz: can't destroy locked resource (pigz.c:2622:mutex_destroy)
unpigz: abort: internal threads error

or

$ ./unpigz -l test.txt.gz
unpigz: skipping: test.txt.gz unrecognized format
unpigz: can't destroy locked resource (pigz.c:2622:mutex_destroy)
unpigz: abort: internal threads error

[1] https://github.com/madler/pigz/issues/96



Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>