Fix integer overflow in device_list_update.go (#1717)

Fix #1511
On 32-bits systems, int(hash.Sum32()) can be negative.
This makes the computation of array indices using modulo invalid, crashing dendrite.
Signed-off-by: Loïck Bonniot <git@lesterpig.com>

keyserver/internal/device_list_update.go

@@ -245,7 +245,7 @@ func (u *DeviceListUpdater) notifyWorkers(userID string) {
 	}
 	hash := fnv.New32a()
 	_, _ = hash.Write([]byte(remoteServer))
-	index := int(hash.Sum32()) % len(u.workerChans)
+	index := int(int64(hash.Sum32()) % int64(len(u.workerChans)))
 
 	ch := u.assignChannel(userID)
 	u.workerChans[index] <- remoteServer