[receiver/windowseventlog] Fix panic when rendering excessively long …

…offset

pkg/stanza/operator/input/windows/buffer.go

@@ -23,6 +23,9 @@ type Buffer struct {
 
 // ReadBytes will read UTF-8 bytes from the buffer, where offset is the number of bytes to be read
 func (b *Buffer) ReadBytes(offset uint32) ([]byte, error) {
+	if offset > uint32(len(b.buffer)) {
+		offset = uint32(len(b.buffer))
+	}
 	utf16 := b.buffer[:offset]
 	utf8, err := unicode.UTF16(unicode.LittleEndian, unicode.UseBOM).NewDecoder().Bytes(utf16)
 	if err != nil {

pkg/stanza/operator/input/windows/buffer_test.go

@@ -21,6 +21,17 @@ func TestBufferReadBytes(t *testing.T) {
 	require.Equal(t, utf8, bytes)
 }
 
+func TestBufferReadBytesOverflow(t *testing.T) {
+	buffer := NewBuffer()
+	utf8 := []byte("test")
+	utf16, _ := unicode.UTF16(unicode.LittleEndian, unicode.UseBOM).NewEncoder().Bytes(utf8)
+	copy(buffer.buffer, utf16)
+	offset := uint32(len(utf16))
+	bytes, err := buffer.ReadBytes(offset * 2)
+	require.NoError(t, err)
+	require.Equal(t, utf8, bytes)
+}
+
 func TestBufferReadWideBytes(t *testing.T) {
 	buffer := NewBuffer()
 	utf8 := []byte("test")