Synchronize message synchronization

app/src/main/java/eu/faircode/email/Core.java

@@ -1027,6 +1027,7 @@ class Core {
             IMAPFolder ifolder, IMAPMessage imessage,
             boolean browsed,
             List<EntityRule> rules) throws MessagingException, IOException {
+        synchronized (folder) {
             long uid = ifolder.getUID(imessage);
 
             if (imessage.isExpunged()) {
@@ -1294,6 +1295,7 @@ class Core {
 
             return message;
         }
+    }
 
     private static void runRules(Context context, IMAPMessage imessage, EntityMessage message, List<EntityRule> rules) {
         if (!Helper.isPro(context))