fix: prevent panic on malformed handshake (#819)

This PR resolves a panic in decodeFirstPart when the functions receives malformed data.
```
panic: runtime error: slice bounds out of range [:2] with capacity 0
```

Author: btoonk

server/handshake_resp.go

@@ -60,8 +60,13 @@ func (c *Conn) readFirstPart() ([]byte, int, error) {
 	return c.decodeFirstPart(data)
 }
 
-func (c *Conn) decodeFirstPart(data []byte) ([]byte, int, error) {
-	pos := 0
+func (c *Conn) decodeFirstPart(data []byte) (newData []byte, pos int, err error) {
+	// prevent 'panic: runtime error: index out of range' error
+	defer func() {
+		if recover() != nil {
+			err = NewDefaultError(ER_HANDSHAKE_ERROR)
+		}
+	}()
 
 	// check CLIENT_PROTOCOL_41
 	if uint32(binary.LittleEndian.Uint16(data[:2]))&CLIENT_PROTOCOL_41 == 0 {

server/handshake_resp_test.go

@@ -28,10 +28,17 @@ func TestReadAuthData(t *testing.T) {
 }
 
 func TestDecodeFirstPart(t *testing.T) {
-	data := []byte{141, 174, 255, 1, 0, 0, 0, 1, 8}
-
 	c := &Conn{}
 
+	// test out of range index returns 'bad handshake' error
+	_, _, err := c.decodeFirstPart([]byte{141, 174})
+	if err == nil || err.Error() != "ERROR 1043 (08S01): Bad handshake" {
+		t.Fatal("expected error, got nil")
+	}
+
+	// test good index position
+	data := []byte{141, 174, 255, 1, 0, 0, 0, 1, 8}
+
 	result, pos, err := c.decodeFirstPart(data)
 	if err != nil {
 		t.Fatalf("expected nil error, got %v", err)