reflect: initialize ptrdata earlier in StructOf

It needs to be set before addTypeBits is called.

Fixes #31043

Change-Id: I692b4047dc17bd68202d45da41dd55d432383e59
Reviewed-on: https://go-review.googlesource.com/c/go/+/169318
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/type.go

@@ -2639,6 +2639,7 @@ func StructOf(fields []StructField) Type {
 	typ.tflag = 0
 	typ.hash = hash
 	typ.size = size
+	typ.ptrdata = typeptrdata(typ.common())
 	typ.align = typalign
 	typ.fieldAlign = typalign
 	typ.ptrToThis = 0
@@ -2709,7 +2710,6 @@ func StructOf(fields []StructField) Type {
 			typ.gcdata = &bv.data[0]
 		}
 	}
-	typ.ptrdata = typeptrdata(typ.common())
 	typ.alg = new(typeAlg)
 	if hashable {
 		typ.alg.hash = func(p unsafe.Pointer, seed uintptr) uintptr {