fix: when multipartform no data,FormValue do not need panic

[alexDango]

no need panic.

[erikdubbelboer]

I don't think this can ever happen? Can you add a test that panics before your change? Thanks.

[alexDango]

sorry, i made a wrong description.
this panic always happen in my server, make my server restart always.

func TestClientGETNothingWithMultipartFormValue(t *testing.T) {
	t.Parallel()
	ln := fasthttputil.NewInmemoryListener()
	realPanic := "no"
	s := &Server{
		Handler: func(ctx *RequestCtx) {
			defer func() {
				if r := recover(); r != nil{
					realPanic = fmt.Sprintf("%v", r)
				}
			}()
			ctx.FormValue("a")
		},
	}
	go s.Serve(ln)

	c := &Client{
		Dial: func(addr string) (net.Conn, error) {
			return ln.Dial()
		},
	}
	req := AcquireRequest()
	res := AcquireResponse()

	req.SetRequestURI("http://example.com")
	req.Header.SetMethod(MethodGet)
	//this is use postman to start a request, use form-data
	req.Header.SetContentType("multipart/form-data;boundary=--------------------------530872294864144413907775")
	err := c.Do(req, res)
	if err != nil {
		t.Fatal(err)
	}
	if realPanic != "no" {
		t.Fatal("realPanic is: ",realPanic)
	}
} 

[erikdubbelboer]

Thanks.