Fix unset method in multidimensional arrays (#105)

A segfault happens when an array with an unset method
is turned into a multidimensional array. Reproducer:
function foo {
    typeset -a a
    a.unset() {
        print unset
    }
    a[3][6][11][20]=7
}
foo

src/cmd/ksh93/sh/nvdisc:
- Fix the multidimensional array unset method crash by
  checking if np->nvenv is an array, since multidimensional
  arrays need to be handled as arrays. This bugfix was
  backported from ksh93v- 2013-10-10-alpha.

src/cmd/ksh93/tests/arrays2.sh:
- Add the reproducer as a regression test for the crash
  with multidimensional arrays.

Bug report on the old mailing list:
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg01195.html

NEWS

@@ -5,6 +5,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2020-08-05:
 
+- Fixed a bug in functions that caused ksh to crash when an array with an
+  unset method was turned into a multidimensional array.
+
 - Fixed a bug that caused scripts to continue running after over-shifting
   in a function when the function call had a redirection.
 

src/cmd/ksh93/sh/nvdisc.c

@@ -331,13 +331,18 @@ static void	assign(Namval_t *np,const char* val,int flags,Namfun_t *handle)
 	else if(!nq || !isblocked(bp,type))
 	{
 		Dt_t *root = sh_subfuntree(1);
+		Namval_t *pp=0;
 		int n;
 		Namarr_t *ap;
 		block(bp,type);
 		nv_disc(np,handle,NV_POP);
+		if(!nv_isattr(np,NV_MINIMAL))
+			pp = (Namval_t*)np->nvenv;
 		nv_putv(np, val, flags, handle);
 		if(sh.subshell)
 			goto done;
+		if(pp && nv_isarray(pp))
+			goto done;
 		if(nv_isarray(np) && (ap=nv_arrayptr(np)) && ap->nelem>0)
 			goto done;
 		for(n=0; n < sizeof(vp->disc)/sizeof(*vp->disc); n++)

src/cmd/ksh93/tests/arrays2.sh

@@ -224,4 +224,21 @@ print -v cx > /dev/null
 print -v cx | read -C l 2> /dev/null || err_exit 'read -C fails from output of print -v'
 [[ ${cx%cx=} ==  "${l%l=}" ]] || err_exit 'print -v for compound variable with fixed 2d array not working'
 
+# ======
+# Multidimensional arrays with an unset method shouldn't cause a crash.
+# The test itself must be run inside of a function.
+multiarray_unset="$tmp/multiarray_unset.sh"
+cat >| "$multiarray_unset" << EOF
+function foo {
+	typeset -a a
+	a.unset() {
+		print unset
+	}
+	a[3][6][11][20]=7
+}
+foo
+EOF
+$SHELL "$multiarray_unset" > /dev/null || err_exit 'Multidimensional arrays with an unset method crash ksh'
+
+# ======
 exit $((Errors<125?Errors:125))