Check push reply ends in a NUL byte

[reynir]

We blindly cut off the last byte

miragevpn/src/engine.ml

Lines 581 to 587 in 8d70130

	let maybe_push_reply config = function
	| Some data ->
	if String.equal "" data then
	Error (`Msg "push request sent: empty TLS reply")
	else
	let str = String.sub data 0 (pred (String.length data)) in
	Log.info (fun m -> m "push request sent, received TLS payload %S" str);

Thinking about it for a bit we should maybe use Cc_message for push replies as well? And maybe the server can send an EXIT message before we receive a push reply and we don't handle that.

[reynir]

Originally reported in #279 (comment)