V555 The expression 'xBlob->length() - nDataWritten > 0' will work as
     'xBlob->length() != nDataWritten'.

Calling XBlob::length() only once allows to avoid overhead of acquiring
mutex per each iteration.

V555 The expression 'xBytes.getLength() - nDataWritten > 0' will work
     as 'xBytes.getLength() != nDataWritten'.

Change-Id: I2e1772fe3af16ac6a6d6f5d2854e84f2fc69ff5f
Reviewed-on: https://gerrit.libreoffice.org/63243
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>