runtime: don't return a slice with nil ptr but non-zero len from growslice

Fixes #10135.

Change-Id: Ic4c5ab15bcb7b9c3fcc685a788d3b59c60c26e1e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7400
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/slice.go

@@ -50,7 +50,9 @@ func growslice(t *slicetype, old sliceStruct, n int) sliceStruct {
 
 	et := t.elem
 	if et.size == 0 {
-		return sliceStruct{old.array, old.len, cap}
+		// append should not create a slice with nil pointer but non-zero len.
+		// We assume that append doesn't need to preserve old.array in this case.
+		return sliceStruct{unsafe.Pointer(&zerobase), old.len, cap}
 	}
 
 	newcap := old.cap