tilde_expand2(): simplify (re: 1829e86)

The nv_putval() and nv_getval() commands are guaranteed to restore
the stack, directly or indirectly. If a discipline shell function
gets executed, the stack is restored by sh_exec() after executing
(see xec.c lines 2633-2636). So the code block handling the
.sh.tilde.* disciplines does not need to.

After this tweak, the tilde.sh regression tests still pass,
including when compiled with ASan.

src/cmd/ksh93/sh/macro.c

@@ -2706,25 +2706,23 @@ static void tilde_expand2(int offset)
 	char		*cp = NULL;			/* character pointer for tilde expansion result */
 	char		*stakp = stkptr(sh.stk,0);	/* current stack object (&stakp[offset] is tilde string) */
 	int		curoff = stktell(sh.stk);	/* current offset of current stack object */
+	sfputc(sh.stk,0);				/* terminate current stack object to avoid data corruption */
 	/*
 	 * Allow overriding tilde expansion with a .sh.tilde.set or .get discipline function.
 	 */
 	if(!sh.tilde_block && SH_TILDENOD->nvfun && SH_TILDENOD->nvfun->disc)
 	{
-		sfputc(sh.stk,'\0');			/* terminate current stack object to avoid data corruption */
 		sh.tilde_block = 1;
 		nv_putval(SH_TILDENOD, &stakp[offset], 0);
 		cp = nv_getval(SH_TILDENOD);
 		sh.tilde_block = 0;
 		if(cp[0]=='\0' || cp[0]=='~')
 			cp = NULL;			/* do not use empty or unexpanded result */
-		stkseek(sh.stk,curoff);			/* restore stack to state on function entry */
 	}
 	/*
 	 * Perform default tilde expansion unless overridden.
 	 * Write the result to the stack, if any.
 	 */
-	sfputc(sh.stk,0);
 	if(!cp)
 		cp = sh_tilde(&stakp[offset]);
 	if(cp)