fdo#75735 RTF import: fix COW when writing a nested structure

Regression from a48e2fd9049797110b3b2505c363557284987ca8 (fdo#44736 -
convert RTFSprms to a copy-on-write structure., 2012-12-07)

(cherry picked from commit 6b6e4d471c2954d34d280398ed1c986d7fb27ae1)

Conflicts:
	writerfilter/source/rtftok/rtfsprm.hxx

Change-Id: I2538f440e29cef6d40db2ea914e4afcbfe411890
Reviewed-on: https://gerrit.libreoffice.org/9100
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: David Tardon <dtardon@redhat.com>