ofz#860 clear old data before reading new data

Change-Id: I3bf5c2072a328052004c4c0551c2b125cb8ab19b
Reviewed-on: https://gerrit.libreoffice.org/35165Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

hwpfilter/source/hwpfile.cxx

@@ -357,6 +357,8 @@ void HWPFile::TagsRead()
                         return;
                      }
 
+                     _hwpInfo.back_info.data.clear();
+
                      //read potentially compressed data in blocks as its more
                      //likely large values are simply broken and we'll run out
                      //of data before we need to realloc