internal/impl: weak field bugfixes

Fix a reversed error check in impl.Export{}.WeakNil.

Check to see if we have a type for the weak field on marshal/size.

Treat a typed nil valued in XXX_Weak as not indicating presence for
the field.

Change-Id: Id667ac7eb4f53236be9e181017082bd8cd21d115
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198717
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/pointer_unsafe.go b/internal/impl/pointer_unsafe.go
index b7f2b1e..201fc2b 100644
--- a/internal/impl/pointer_unsafe.go
+++ b/internal/impl/pointer_unsafe.go
@@ -110,7 +110,7 @@
 func (p pointer) StringSlice() *[]string                { return (*[]string)(p.p) }
 func (p pointer) Bytes() *[]byte                        { return (*[]byte)(p.p) }
 func (p pointer) BytesSlice() *[][]byte                 { return (*[][]byte)(p.p) }
-func (p pointer) WeakFields() *WeakFields               { return (*WeakFields)(p.p) }
+func (p pointer) WeakFields() *weakFields               { return (*weakFields)(p.p) }
 func (p pointer) Extensions() *map[int32]ExtensionField { return (*map[int32]ExtensionField)(p.p) }
 
 func (p pointer) Elem() pointer {