Fix unable to vote with ConcurrentVoteException when ValueMax is null

Closes #276
Signed-off-by: Thomas Citharel <tcit@tcit.fr>

app/classes/Framadate/Services/PollService.php

@@ -296,7 +296,7 @@ class PollService {
         $best_choices = $this->computeBestChoices($votes);
         foreach ($best_choices['y'] as $i => $nb_choice) {
             // if for this option we have reached maximum value and user wants to add itself too
-            if ($nb_choice >= $poll->ValueMax && $user_choice[$i] === "2") {
+	     if ($poll->ValueMax != null && $nb_choice >= $poll->ValueMax && $user_choice[$i] === "2") {
                 throw new ConcurrentVoteException();
             }
         }