Add a test for memclr.

internal/buffer/out_message_test.go

 package buffer
 
 import (
+	"crypto/rand"
 	"fmt"
+	"io"
 	"testing"
 	"unsafe"
 )
 
+func randBytes(n int) (b []byte, err error) {
+	b = make([]byte, n)
+	_, err = io.ReadFull(rand.Reader, b)
+	return
+}
+
+func TestMemclr(t *testing.T) {
+	// All sizes up to 32 bytes.
+	var sizes []int
+	for i := 0; i <= 32; i++ {
+		sizes = append(sizes, i)
+	}
+
+	// And a few hand-chosen sizes.
+	sizes = append(sizes, []int{
+		39, 41, 64, 127, 128, 129,
+		1<<20 - 1,
+		1 << 20,
+		1<<20 + 1,
+	}...)
+
+	// For each size, fill a buffer with random bytes and then zero it.
+	for _, size := range sizes {
+		size := size
+		t.Run(fmt.Sprintf("size=%d", size), func(t *testing.T) {
+			// Generate
+			b, err := randBytes(size)
+			if err != nil {
+				t.Fatalf("randBytes: %v", err)
+			}
+
+			// Clear
+			var p unsafe.Pointer
+			if len(b) != 0 {
+				p = unsafe.Pointer(&b[0])
+			}
+
+			memclr(p, uintptr(len(b)))
+
+			// Check
+			for i, x := range b {
+				if x != 0 {
+					t.Fatalf("non-zero byte %d at offset %d", x, i)
+				}
+			}
+		})
+	}
+}
+
 func BenchmarkOutMessageReset(b *testing.B) {
 	// A single buffer, which should fit in some level of CPU cache.
 	b.Run("Single buffer", func(b *testing.B) {