eve/drop: use highest priority drop

When adding the alert to a drop record make sure the add the highest
priority.

It would until now add all drops from high to low prio, effectively
overwriting the record each time.

Ticket #4397

src/output-json-drop.c

@@ -164,6 +164,7 @@ static int DropLogJSON (JsonDropLogThread *aft, const Packet *p)
             {
                 AlertJsonHeader(NULL, p, pa, js, 0, &addr);
                 logged = 1;
+                break;
             }
         }
         if (logged == 0) {