[aot] Fix a memory leak related to constraint allocation/deallocation. (

#4880)

* [jit] Make a copy of the constraint in mini_get_shared_gparam().

* [aot] Free the temporary gshared constraint in decode_class_ref().

mono/mini/aot-runtime.c

@@ -505,6 +505,7 @@ decode_klass_ref (MonoAotModule *module, guint8 *buf, guint8 **endbuf, MonoError
 				return NULL;
 
 			t = mini_get_shared_gparam (&par_klass->byval_arg, gshared_constraint);
+			mono_metadata_free_type (gshared_constraint);
 			klass = mono_class_from_mono_type (t);
 		} else {
 			int type = decode_value (p, &p);

mono/mini/mini-generic-sharing.c

@@ -3498,6 +3498,7 @@ mini_get_shared_gparam (MonoType *t, MonoType *constraint)
 	copy = (MonoGSharedGenericParam *)mono_image_alloc0 (image, sizeof (MonoGSharedGenericParam));
 	memcpy (&copy->param, par, sizeof (MonoGenericParamFull));
 	copy->param.info.pklass = NULL;
+	constraint = mono_metadata_type_dup (image, constraint);
 	name = get_shared_gparam_name (constraint->type, ((MonoGenericParamFull*)copy)->info.name);
 	copy->param.info.name = mono_image_strdup (image, name);
 	g_free (name);