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 /