Fix memory corruption when a compound variable is unset (#49)

The following set of commands ends with a memory fault under
certain circumstances because ksh attempts to free memory
twice, causing memory corruption:

$ testarray=(1 2)
$ compound testarray
$ unset testarray
$ eval testarray=

The fix is to make sure 'np->nvfun' is a valid pointer before
attempting to free memory in 'put_tree'. This patch is from
OpenSUSE: https://build.opensuse.org/package/view_file/shells/ksh/ksh93-nvtree-free.dif?expand=1

src/cmd/ksh93/sh/nvtree.c:
- Do not try to free memory when 'np->nvfun' and 'val'
  are false.

src/cmd/ksh93/tests/comvar.sh:
- Add a regression test for the double free problem. The
  reproducer must be run from an executable script
  with 'ksh -c'.

src/cmd/ksh93/sh/nvtree.c

@@ -1142,6 +1142,8 @@ static void put_tree(register Namval_t *np, const char *val, int flags,Namfun_t
 	nv_putv(np, val, flags,fp);
 	if(val && nv_isattr(np,(NV_INTEGER|NV_BINARY)))
 		return;
+	if(!val && !np->nvfun)
+		return;
 	if(ap= nv_arrayptr(np))
 		nleft = array_elem(ap);
 	if(nleft==0)