tar: no need to report getcwd error if never using the result

tar: no need to report getcwd error if never using the result

* src/misc.c (struct wd): Rename 'saved' to 'err', with new semantics.
(chdir_arg, chdir_do): Adjust to new semantics.  Do not report an
error merely because save_cwd fails; report an error only if
save_cwd's result is needed later.
* tests/extrac09.at: New file, to test for bug that was fixed.
* tests/testsuite.at: Include it.
* tests/Makefile.am (TESTSUITE_AT): Add it.