Fix crash on subshell exit if PWD is inaccessible (re: dd9bc229)

This commit also further mitigates the problems with restoring an
inaccessible or nonexistent PWD on exiting a virtual subshell.

Harald van Dijk writes:
> On a build of ksh with -fsanitize=undefined to help diagnose
> problems:
>
> $ mkdir deleted
> $ cd deleted
> $ rmdir ../deleted
> $ ksh -c '(cd /; (cd /)); :'
> /home/harald/ksh/src/cmd/ksh93/sh/subshell.c:561:22: runtime
> error: null pointer passed as argument 1, which is declared to
> never be null
> Segmentation fault (core dumped)
>
> Note that it segfaults the same with default compilation flags,
> but it does not print out the useful extra message. The code
> assumes that pwd is non-null and passes it to strcmp without
> checking, but it will be null if the current directory cannot be
> determined, for instance because it has been deleted.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Avoid the null pointer dereference reported above.

src/cmd/ksh93/bltins/cd_pwd.c: b_cd():
- Fork a virtual subshell even on systems with fchdir(2) if the
  present working directory tests as inaccessible on invoking 'cd';
  it may no longer exist and fchdir would fail to get a handle.
  (For the test we have to opendir(3) the full path to the PWD and
  not ".", as the latter may succeed even if the PWD is gone.)

src/cmd/ksh93/data/builtins.c:
- Update 'cd' version string.

Fixes:   https://github.com/ksh93/ksh/issues/153
Related: https://github.com/ksh93/ksh/issues/141

src/cmd/ksh93/tests/subshell.sh

@@ -893,5 +893,17 @@ got=$( { "$SHELL" "$tmp/crash_rhbz1117404.ksh"; } 2>&1)
 ((!(e = $?))) || err_exit 'crash while handling subshell trap' \
 	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
 
+# ======
+# Segmentation fault when using cd in a subshell, when current directory cannot be determined
+# https://github.com/ksh93/ksh/issues/153
+cd "$tmp"
+mkdir deleted
+cd deleted
+tmp=$tmp "$SHELL" -c 'cd /; rmdir "$tmp/deleted"'
+exp="PWD=$PWD"
+got=$( { "$SHELL" -c '(cd /; (cd /)); print -r -- "PWD=$PWD"'; } 2>&1 )
+((!(e = $?))) && [[ $got == "$exp" ]] || err_exit 'failed to restore nonexistent PWD on exiting a virtual subshell' \
+	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))