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.

src/unlink.c

@@ -86,8 +86,9 @@ flush_deferred_unlinks (bool force)
 	    {
 	      const char *fname;
 
-	      if (p->file_name[0] == 0 ||
-		  strcmp (p->file_name, ".") == 0)
+	      if (p->dir_idx
+		  && (p->file_name[0] == 0
+		      || strcmp (p->file_name, ".") == 0))
 		{
 		  fname = tar_dirname ();
 		  chdir_do (p->dir_idx - 1);

tests/Makefile.am

@@ -187,6 +187,7 @@ TESTSUITE_AT = \
  remfiles09a.at\
  remfiles09b.at\
  remfiles09c.at\
+ remfiles10.at\
  same-order01.at\
  same-order02.at\
  shortfile.at\

tests/remfiles10.at

@@ -0,0 +1,46 @@
+# Process this file with autom4te to create testsuite. -*- Autotest -*-
+# Test suite for GNU tar.
+# Copyright 2014 Free Software Foundation, Inc.
+#
+# GNU tar is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# GNU tar is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check --remove-files with .
+
+AT_SETUP([remove-files])
+AT_KEYWORDS([create remove-files remfiles10])
+
+AT_TAR_CHECK([
+mkdir foo
+echo foo/file > foo/file
+decho A
+(cd foo && tar -cvf ../foo.tar --remove-files .)
+tar_status=$?
+decho B
+find foo
+exit $tar_status
+],
+[2],
+[A
+./
+./file
+B
+foo
+],
+[A
+tar: .: Cannot rmdir: Invalid argument
+tar: Exiting with failure status due to previous errors
+B
+],[],[],[gnu])
+
+AT_CLEANUP

tests/testsuite.at

@@ -399,6 +399,7 @@ m4_include([remfiles08c.at])
 m4_include([remfiles09a.at])
 m4_include([remfiles09b.at])
 m4_include([remfiles09c.at])
+m4_include([remfiles10.at])
 
 AT_BANNER([Extended attributes])
 m4_include([xattr01.at])