obj: fix bug in marshal of nulls in []interface{}.

Yes, []interface{} *specifically*.  For any other []*ConcreteT, this bug would not strike.

Never let anyone tell you that reflection is easy nor fun.  It's a maze of twisty little passages and you are extremely likely to be eaten by a grue.

Signed-off-by: Eric Myhre <hash@exultant.us>

obj/marshalWildcard.go

@@ -18,6 +18,7 @@ type marshalMachineWildcard struct {
 func (mach *marshalMachineWildcard) Reset(slab *marshalSlab, rv reflect.Value, rt reflect.Type) error {
 	// If the interface contains nil, go no further; we'll simply yield that single token.
 	if rv.IsNil() {
+		mach.delegate = nil
 		return nil
 	}
 	// Pick, reset, and retain a delegate machine for the interior type.

obj/objFixtures_test.go

@@ -1068,6 +1068,8 @@ var objFixtures = []struct {
 	{title: "nulls in midst of arrays",
 		sequence: fixtures.SequenceMap["null in middle of array"],
 		marshalResults: []marshalResults{
+			{title: "from []iface",
+				valueFn: func() interface{} { return []interface{}{"one", nil, "three", nil, "five"} }},
 			{title: "from []*str",
 				valueFn: func() interface{} {
 					one, three, five := "one", "three", "five"