nv_disc(NV_LAST) loses trailing shell context from discipline stack (#276)

This is the underlying cause for the issue worked around in
3654ee73.

The following explanation refers to the current illumos version of
ksh93 and shows output from illumos' modular debugger:
https://illumos.org/books/dev/debugging.html

Each environment variable (name/value pair) has a linked list of
disciplines attached to it, and at the end of that list there is
optionally a shell context pointer. For example, for the EDITOR
variable:

        > ::bp libshell.so.1`put_ed
        > ::run
        $
        $ EDITOR=vim
        > ::stack ! head -1
        libshell.so.1`put_ed+0x14(e06208, e01c58, 0, dced90)
        > e06208::print Namval_t
        {
            nvname = 0xfffffbffeec40a0e "EDITOR"
            nvfun = 0xdced90
            nvalue = 0
        }
        > e06208::print Namval_t nvfun | ::print Namfun_t
        {
            disc = libshell.so.1`EDITOR_disc
            next = libshell.so.1`sh+0x710
        }

Here, the EDITOR Namval_t has a discipline stack containing
EDITOR_disc and &Shell_t.nvfun.

The problem arises when a new discipline is pushed onto the stack,
such as when using typeset -u to add an upper-case translation
discipline.

    $ typeset -u EDITOR
    > e06208::print Namval_t
    {
        nvname = 0xfffffbffeec40a0e "EDITOR"
        nvfun = 0xdced90
        nvalue = 0xe0fdb0 "vim"
    }
    > e06208::print Namval_t nvfun | ::print Namfun_t
    {
        disc = libshell.so.1`EDITOR_disc
        next = 0xdc27a0
    }
    > e06208::print Namval_t nvfun | ::print Namfun_t next | ::print Namfun_t
    {
        disc = libshell.so.1`TRANS_disc
        next = 0
    }

TRANS_disc has been pushed onto the end of the discipline stack,
but the shell handle has been lost.

With this change, the attributes and variables tests pass (this is
on illumos where this change originates).

src/cmd/ksh93/sh/nvdisc.c

@@ -752,7 +752,7 @@ Namfun_t *nv_disc(register Namval_t *np, register Namfun_t* fp, int mode)
 			if(lp && !lp->disc)
 				fp->next = lp;
 			else
-				fp->next = 0;
+				fp->next = *lpp;
 		}
 		else
 		{