Actually deactivate CDPATH when unsetting it

After 'unset CDPATH', CDPATH continued to work as if nothing
happened. Unsetting it should be a valid way to deactivate it.

This bug is in every ksh93 version.

src/cmd/ksh93/bltins/cd_pwd.c: b_cd():
- Fix a manifest logic error: first check if CDPATH (CDPNOD) is
  unset before assigning to 'cdpath', not the other way around.
  Setting the 'cdpath' pointer is what activates the CDPATH search.

NEWS

@@ -5,6 +5,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2021-12-28:
 
+- Fixed a bug that caused CDPATH to continue working after unsetting it.
+
 - Added three options to the ulimit builtin with the same names and
   functionality as in Bash:
   - 'ulimit -k' sets the maximum number of kqueues.

src/cmd/ksh93/bltins/cd_pwd.c

@@ -140,7 +140,7 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	&& !(dir[0]=='.' && (dir[1]=='/' || dir[1]==0))
 	&& !(dir[0]=='.' && dir[1]=='.' && (dir[2]=='/' || dir[2]==0)))
 	{
-		if(!(cdpath = (Pathcomp_t*)shp->cdpathlist) && (dp=sh_scoped(shp,CDPNOD)->nvalue.cp))
+		if((dp=sh_scoped(&sh,CDPNOD)->nvalue.cp) && !(cdpath = (Pathcomp_t*)shp->cdpathlist))
 		{
 			if(cdpath=path_addpath(shp,(Pathcomp_t*)0,dp,PATH_CDPATH))
 			{

src/cmd/ksh93/tests/variables.sh

@@ -305,6 +305,8 @@ x=$(cd ${tmp#/})
 if	[[ $x != $tmp ]]
 then	err_exit "CDPATH ${tmp#/} does not display new directory"
 fi
+unset CDPATH
+cd "${tmp#/}" >/dev/null 2>&1 && err_exit "CDPATH not deactivated after unset"
 cd "$tmp" || exit
 TMOUT=100
 (TMOUT=20)