PATCH: Avoid setting flags if IrpContext was set to NULL.in FFSExceptionFilter()

When the Identifier Type of the IrpContext is not FFSICX or when the Identifier Size is not set properly, we are setting IrpContext to Null.

Shortly afterwards we try to SetFlag to a valid IrpContext

This patch checks if IrpContext is not NULL before trying to mark this request as "Wait-able".

CID 1363522
CID 1363631