Rename structPointer_Map to structPointer_NewAt.

It was never map-specific, and will soon be used for other types.
diff --git a/proto/decode.go b/proto/decode.go
index 312e604..bf71dca 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -675,7 +675,7 @@
 	oi := o.index       // index at the end of this map entry
 	o.index -= len(raw) // move buffer back to start of map entry
 
-	mptr := structPointer_Map(base, p.field, p.mtype) // *map[K]V
+	mptr := structPointer_NewAt(base, p.field, p.mtype) // *map[K]V
 	if mptr.Elem().IsNil() {
 		mptr.Elem().Set(reflect.MakeMap(mptr.Type().Elem()))
 	}