reflect: zero stack slots before writing to them with write barriers

reflect.assignTo writes to the target using write barriers. Make sure
that the memory it is writing to is zeroed, so the write barrier does
not read pointers from uninitialized memory.

Fixes #39541

Change-Id: Ia64b2cacc193bffd0c1396bbce1dfb8182d4905b
Reviewed-on: https://go-review.googlesource.com/c/go/+/238760
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/stack.go

@@ -556,6 +556,7 @@ func adjustpointer(adjinfo *adjustinfo, vpp unsafe.Pointer) {
 }
 
 // Information from the compiler about the layout of stack frames.
+// Note: this type must agree with reflect.bitVector.
 type bitvector struct {
 	n        int32 // # of bits
 	bytedata *uint8