MDL-74394 mnet: Better check for openssl_get_publickey() failure

In PHP8, this func now returns an object of type OpenSSLAsymmetricKey, not a
resource.

Reference:
https://www.php.net/manual/en/function.openssl-pkey-get-public.php

mnet/lib.php

@@ -259,7 +259,7 @@ function mnet_encrypt_message($message, $remote_certificate) {
     // Generate a key resource from the remote_certificate text string
     $publickey = openssl_get_publickey($remote_certificate);
 
-    if ( gettype($publickey) != 'resource' ) {
+    if ($publickey === false) {
         // Remote certificate is faulty.
         return false;
     }