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

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))