tar: fix --verify option, which broke in 1.24

* NEWS: Document this.
* src/compare.c (verify_volume): Decode the header before invoking
diff_archive, as diff_archive no longer does this as of the
2010-06-28 commit.  Also, don't try to invoke diff_archive on a
zero block.
* tests/Makefile.am (TESTSUITE_AT): Add verify.at.
* tests/testsuite.at: Include verify.at.
* tests/verify.at: New file.

src/compare.c

@@ -611,8 +611,10 @@ verify_volume (void)
 		       (0, 0, _("A lone zero block at %s"),
 			STRINGIFY_BIGINT (current_block_ordinal (), buf)));
             }
+	  continue;
 	}
 
+      decode_header (current_header, &current_stat_info, &current_format, 1);
       diff_archive ();
       tar_stat_destroy (&current_stat_info);
     }