cd: fix an invalid free() call (Solaris patch 211-21547336)

This change is pulled from here:
https://github.com/oracle/solaris-userland/blob/master/components/ksh93/patches/211-21547336.patch

src/cmd/ksh93/bltins/cd_pwd.c: b_cd():
- The functions path_pwd() and path_relative() in sh/path.c may
  return a pointer to e_dot[] (".") as a fallback if they fail to
  determine a path. This is a string in read-only memory
  (data/msg.c), so must not be freed. A pointer to that string may
  end up in sh.pwd (== shp->pwd), so b_cd() needs a check for that.

src/cmd/ksh93/bltins/cd_pwd.c

@@ -212,7 +212,7 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	nv_scan(sh_subtracktree(1),rehash,(void*)0,NV_TAGGED,NV_TAGGED);
 	path_newdir(shp,shp->pathlist);
 	path_newdir(shp,shp->cdpathlist);
-	if(oldpwd)
+	if(oldpwd && oldpwd!=e_dot)
 		free(oldpwd);
 	return(0);
 }