Io.net: set receive{,Timeout} message to init

If we use `undefined`, then `netReceive` can `@intCast` the
control slice len to msghdr controllen, which is sometimes `u32`,
even on 64-bit platforms.

`init` just avoids this entirely by setting `control` to an empty
slice rather than undefined.

Author: Rexicon226

lib/std/Io/net.zig

@@ -1118,7 +1118,7 @@ pub const Socket = struct {
     /// See also:
     /// * `receiveTimeout`
     pub fn receive(s: *const Socket, io: Io, buffer: []u8) ReceiveError!IncomingMessage {
-        var message: IncomingMessage = undefined;
+        var message: IncomingMessage = .init;
         const maybe_err, const count = io.vtable.netReceive(io.userdata, s.handle, (&message)[0..1], buffer, .{}, .none);
         if (maybe_err) |err| switch (err) {
             // No timeout is passed to `netReceieve`, so it must not return timeout related errors.
@@ -1144,7 +1144,7 @@ pub const Socket = struct {
         buffer: []u8,
         timeout: Io.Timeout,
     ) ReceiveTimeoutError!IncomingMessage {
-        var message: IncomingMessage = undefined;
+        var message: IncomingMessage = .init;
         const maybe_err, const count = io.vtable.netReceive(io.userdata, s.handle, (&message)[0..1], buffer, .{}, timeout);
         if (maybe_err) |err| return err;
         assert(1 == count);