RFR(XXS): 8242485: Null _file checking in fileStream::flush()

[Denghui Dong]

Hi team,

Could you please review this small patch?

JBS: https://bugs.openjdk.java.net/browse/JDK-8242485
webrev: http://cr.openjdk.java.net/~ddong/8242485/webrev.00/

I found the method "fileStream::flush()" lacks null check before fflush  which will cause
all open output streams are flushed by fflush() if _file is NULL.

Thanks,
Denghui Dong