Bug #60977 writing XSSF with custom properties twice corrupts output

Bug #60977 writing XSSF with custom properties twice corrupts output

turns out the output XML data buffer for the custom properties document was appended to, not replaced, when writing in the presence of existing content.  Now clearing output buffer first in this case.  When first creating custom properties, i.e. first write after changes, there is no buffer yet, so nothing to clear.

Does not affect any read operations, or other writes beyond custom properties, as that was where the problem lay, not down in the Zip stuff.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1851084 13f79535-47bb-0310-9956-ffa450edef68