[release-branch.go1.6] net/http: multipart ReadForm close file after copy

Always close the file regardless of whether the copy succeeds or fails.
Pass along the close error if the copy succeeds

Updates #16296

Change-Id: Ib394655b91d25750f029f17b3846d985f673fb50
Reviewed-on: https://go-review.googlesource.com/30410Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/33640Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/mime/multipart/formdata.go

@@ -75,8 +75,10 @@ func (r *Reader) ReadForm(maxMemory int64) (f *Form, err error) {
 			if err != nil {
 				return nil, err
 			}
-			defer file.Close()
 			_, err = io.Copy(file, io.MultiReader(&b, p))
+			if cerr := file.Close(); err == nil {
+				err = cerr
+			}
 			if err != nil {
 				os.Remove(file.Name())
 				return nil, err