Fix crash when cd-ing away from a directory that's been removed.

Tickled by Aboriginal Linux.

Addresses issue #58.

core/builtin.py

@@ -589,8 +589,12 @@ def Cd(argv, mem, dir_stack):
      # can't even set it at all.)
      raise AssertionError('Invalid OLDPWD')

  # Save OLDPWD.
  state.SetGlobalString(mem, 'OLDPWD', os.getcwd())
  # NOTE: We can't call os.getcwd() because it can raise OSError if the
  # directory was removed (ENOENT.)

  pwd = mem.GetVar('PWD')
  assert pwd.tag == value_e.Str, pwd  # TODO: Need a general scheme to avoid
  state.SetGlobalString(mem, 'OLDPWD', pwd.s)
  try:
    os.chdir(dest_dir)
  except OSError as e:

spec/builtins.test.sh

@@ -41,6 +41,19 @@ echo status=$?
# stdout: status=1
# OK dash/mksh stdout: status=2

### cd away from dir that was deleted
dir=$TMP/cd-nonexistent
mkdir -p $dir
cd $dir
rmdir $dir
cd $TMP
echo $(basename $OLDPWD)
echo status=$?
## STDOUT:
cd-nonexistent
status=0
## END

### pushd/popd
set -o errexit
cd /