Fix crash on cd in subshell with PWD unset (re: 5ee290c)

Reproducer:

$ ksh -c 'unset PWD; (cd /); :'
Memory fault

The shell crashes because b_cd() is testing the value of the PWD
variable without checking if there is one.

src/cmd/ksh93/sh/path.c: path_pwd():
- Never return an unfreeable pointer to e_dot; always return a
  freeable pointer. This fixes another corner-case crashing bug.
- Make sure the PWD variable gets assigned a value if it doesn't
  have one, even if it's the "." fallback. However, if the PWD is
  inaccessible but we did inherit a $PWD value that starts with a
  /, then use the existing $PWD value as this will help the shell
  fail gracefully.

src/cmd/ksh93/bltins/cd_pwd.c:
- b_cd(): When checking if the PWD is valid, use the sh.pwd copy
  instead of the PWD variable. This fixes the crash above.
- b_cd(): Since path_pwd() now always returns a freeable value,
  free sh.pwd unconditionally before setting the new value.
- b_pwd(): Not only check that path_pwd() returns a value starting
  with a slash, but also verify it with test_inode() and error out
  if it's wrong. This makes the 'pwd' command useful for checking
  that the PWD is currently accessible.

src/cmd/ksh93/data/msg.c:
- Change e_pwd error message for accuracy and clarity.

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-02-16"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-02-17"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */