comments claim return is len of data, but negative numbers for compressed data errors
are returned. Make things return the amount of successful data read and turn into
unsigned size_t to match ofz#727

Change-Id: Iadddfae67bb5d4960ddf82e169fc2592ded85950
Reviewed-on: https://gerrit.libreoffice.org/35077Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>