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/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-01-08"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-01-19"	/* must be in this format for $((.sh.version)) */
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */
 /* Arithmetic $((.sh.version)) uses the last 10 chars, so the date must be at the end. */