lib/mmap: fix lint error on deprecated reflect.SliceHeader

reflect.SliceHeader is deprecated, however the replacement gives a go
vet warning so this disables the lint warning in one use of
reflect.SliceHeader and replaces it in the other.

lib/mmap/mmap_windows.go

@@ -21,20 +21,29 @@ func Alloc(size int) ([]byte, error) {
 	if err != nil {
 		return nil, fmt.Errorf("mmap: failed to allocate memory for buffer: %w", err)
 	}
+	// SliceHeader is deprecated...
 	var mem []byte
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem))
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem)) // nolint:staticcheck
 	sh.Data = p
 	sh.Len = size
 	sh.Cap = size
 	return mem, nil
+	// ...However the correct code gives a go vet warning
+	// "possible misuse of unsafe.Pointer"
+	//
+	// Maybe there is a different way of writing this, but none of
+	// the allowed uses of unsafe.Pointer seemed to cover it other
+	// than using SliceHeader (use 6 of unsafe.Pointer).
+	//
+	// return unsafe.Slice((*byte)(unsafe.Pointer(p)), size), nil
 }
 
 // Free frees buffers allocated by Alloc.  Note it should be passed
 // the same slice (not a derived slice) that Alloc returned.  If the
 // free fails it will return with an error.
 func Free(mem []byte) error {
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&mem))
-	err := windows.VirtualFree(sh.Data, 0, windows.MEM_RELEASE)
+	p := unsafe.SliceData(mem)
+	err := windows.VirtualFree(uintptr(unsafe.Pointer(p)), 0, windows.MEM_RELEASE)
 	if err != nil {
 		return fmt.Errorf("mmap: failed to unmap memory: %w", err)
 	}