Do not use a world-writable /var/crash with the dumper script

Suggested-by: lintian
Fixes: https://bugs.debian.org/924397
See-also: https://bugs.debian.org/515211


Reported-by: Jakub Wilk <jwilk@jwilk.net>
Reported-in: <20190312145027.7npetzqs3hwlkhfr@jwilk.net>