tar: do not dereference NULL pointer with '--remove-files .'

Problem reported by Thorsten Hirsch in:
http://lists.gnu.org/archive/html/bug-tar/2014-04/msg00011.html
* src/unlink.c (flush_deferred_unlinks):
Do not attempt to find the parent of "." when "." is
at the top level.
* tests/remfiles10.at: New file.
* tests/Makefile.am (TESTSUITE_AT):
* tests/testsuite.at: Add it.