patch 9.0.1095: using freed memory when declaration fails

Problem:    Using freed memory when declaration fails. (Yegappan Lakshmanan)
Solution:   After unreferencing an object set the reference to NULL.

src/testdir/test_vim9_class.vim

@@ -349,6 +349,22 @@ def Test_class_object_member_access()
       assert_equal('make = 123', c2.GetMake())
   END
   v9.CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+
+      class MyCar
+        this.make: string
+
+        def new(make_arg: string)
+            this.make = make_arg
+        enddef
+      endclass
+
+      var c = MyCar.new("abc")
+      var c = MyCar.new("def")
+  END
+  v9.CheckScriptFailure(lines, 'E1041:')
 enddef
 
 def Test_class_member_access()

src/typval.c

@@ -162,9 +162,11 @@ clear_tv(typval_T *varp)
 		break;
 	    case VAR_CLASS:
 		class_unref(varp->vval.v_class);
+		varp->vval.v_class = NULL;
 		break;
 	    case VAR_OBJECT:
 		object_unref(varp->vval.v_object);
+		varp->vval.v_object = NULL;
 		break;
 	    case VAR_UNKNOWN:
 	    case VAR_ANY:

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1095,
 /**/
     1094,
 /**/