Eleven: Fix potential NPE when media server crashes

If the service doesn't exist when the media server crashes, Eleven
will crash when creating the TrackErrorInfo object. The SERVER_DIED
message is processed only when the service is not null, so just
ignore the MEDIA_ERROR_SERVER_DIED message if service is null.

BUGBASH-1045

Change-Id: Idaf3e238eefd412ebb85244b48ad7b20bc049ff0