reflect: fix typeptrdata

We can't use ptrdata inside of typeptrdata, because it won't be
properly initialized until typeptrdata returns.

Fixes #31039

Change-Id: Ib8c89191a7e4cce678a05d351bb6ded81ba23aae
Reviewed-on: https://go-review.googlesource.com/c/go/+/169317
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/reflect/type.go

@@ -2775,20 +2775,20 @@ func runtimeStructField(field StructField) structField {
 // containing pointer data. Anything after this offset is scalar data.
 // keep in sync with ../cmd/compile/internal/gc/reflect.go
 func typeptrdata(t *rtype) uintptr {
-	if !t.pointers() {
-		return 0
-	}
 	switch t.Kind() {
 	case Struct:
 		st := (*structType)(unsafe.Pointer(t))
 		// find the last field that has pointers.
-		field := 0
+		field := -1
 		for i := range st.fields {
 			ft := st.fields[i].typ
 			if ft.pointers() {
 				field = i
 			}
 		}
+		if field == -1 {
+			return 0
+		}
 		f := st.fields[field]
 		return f.offset() + f.typ.ptrdata