fix #1904

/dev/null permissions were modified when using sudo rights.
This fixes this bug during decompression.

More importantly, this patch  adds a test, triggered in TravisCI,
ensuring unaltered /dev/null permissions.
diff --git a/.travis.yml b/.travis.yml
index e17dba6..ec1adab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -34,7 +34,8 @@
 
     - name: make test (complete)
       script:
-        - make test
+        # DEVNULLRIGHTS : will request sudo rights to test permissions on /dev/null
+        - DEVNULLRIGHTS=test make test
 
     - name: gcc-6 + gcc-7 compilation
       script: