pkg/vm/stack: make some use of testReadInt64()

GolangCI complains:
  testReadInt64 is unused (from deadcode)

Fix it to always provide correctly-sized buffer for the binary.Read().

pkg/vm/stack/int_test.go

@@ -64,6 +64,8 @@ func TestByteArrConversion(t *testing.T) {
 	ba, err := a.ByteArray()
 	assert.Nil(t, err)
 
+	assert.Equal(t, num, testReadInt64(ba.val))
+
 	have, err := ba.Integer()
 	assert.Nil(t, err)
 

pkg/vm/stack/test_helper.go

@@ -38,7 +38,11 @@ func testMakeStackInt(t *testing.T, num int64) *Int {
 
 func testReadInt64(data []byte) int64 {
 	var ret int64
-	buf := bytes.NewBuffer(data)
+	var arr [8]byte
+
+	// expands or shrinks data automatically
+	copy(arr[:], data)
+	buf := bytes.NewBuffer(arr[:])
 	binary.Read(buf, binary.LittleEndian, &ret)
 	return ret
 }