patch 8.2.3029: Vim9: crash when using operator and list unpack assignment

Problem:    Vim9: crash when using operator and list unpack assignment.
            (Naohiro Ono)
Solution:   Get variable value before operation. (closes #8416)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 1cbdcfb..1d3c20e 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -283,6 +283,29 @@
     [v1, v2; _] = [1, 2, 3, 4, 5]
     assert_equal(1, v1)
     assert_equal(2, v2)
+
+    var a = 1
+    var b = 3
+    [a, b] += [2, 4]
+    assert_equal(3, a)
+    assert_equal(7, b)
+
+    [a, b] -= [1, 2]
+    assert_equal(2, a)
+    assert_equal(5, b)
+
+    [a, b] *= [3, 2]
+    assert_equal(6, a)
+    assert_equal(10, b)
+
+    [a, b] /= [2, 4]
+    assert_equal(3, a)
+    assert_equal(2, b)
+
+    [a, b] = [17, 15]
+    [a, b] %= [5, 3]
+    assert_equal(2, a)
+    assert_equal(0, b)
   END
   CheckDefAndScriptSuccess(lines)