Server panics when tring to access a non-existing form file

[fvln]

Issue Description

Function FormFile tries to close a file handle, even if the file could not be opened. Bug introduced by #1411

Checklist

• [ X ] Dependencies installed
• [ X ] No typos
• [ X ] Searched existing issues and docs

Expected behaviour

Function should return a non-nil error.

Actual behaviour

Panic

Steps to reproduce

package main

func main() {
	e := echo.New()
	e.POST("/uploads", uploadHandler)
}

func uploadHandler(context echo.Context) error {

	file, err := context.FormFile("file")
	if err != nil {
		log.Println("No file uploaded", err)
		return context.String(http.StatusBadRequest, "File is missing")
	}

// Do stuff with returned file

	return context.NoContent(http.StatusCreated)
}

Then run: curl -X POST "http://127.0.0.1:1323/api/v1/uploads" to trigger a panic

server | echo: http: panic serving 192.168.101.1:58638: runtime error: invalid memory address or nil pointer dereference
server | goroutine 54 [running]:
server | net/http.(*conn).serve.func1(0xc420d620a0)
server | 	/usr/lib/go-1.10/src/net/http/server.go:1726 +0xd0
server | panic(0x7c45e0, 0xc34ea0)
server | 	/usr/lib/go-1.10/src/runtime/panic.go:502 +0x229
server | github.com/labstack/echo.(*context).FormFile(0xc4206a0120, 0x83c1d5, 0x4, 0x0, 0x0, 0x0)
server | 	/home/user/go/src/github.com/labstack/echo/context.go:351 +0x62

Version/commit

754b449

Suggested fix

f, fh, err := c.request.FormFile(name)
if f != nil {
	defer f.Close()
}

[vishr]

@fvln Do you mind sending a PR?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cperkkk]

please re-open.