internal/socket: don't crash with corrupted control messages

Change-Id: I5c484662add4261f504607758521718f4c760375
Reviewed-on: https://go-review.googlesource.com/45294
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/socket/socket.go

@@ -175,6 +175,9 @@ func (m ControlMessage) Parse() ([]ControlMessage, error) {
 	for len(m) >= controlHeaderLen() {
 		h := (*cmsghdr)(unsafe.Pointer(&m[0]))
 		l := h.len()
+		if l <= 0 {
+			return nil, errors.New("invalid header length")
+		}
 		if uint64(l) < uint64(controlHeaderLen()) {
 			return nil, errors.New("invalid message length")
 		}