i#74153 RTF import: fix track changes vs superscript char prop

Since 9dbf817fe5c5253fba0831aefa17575ae0ba3af1 (handle scope of
w:pPrChange and w:rPrChange properly (bnc#821804), 2014-10-01), this
caused an assertion failure -> crash.

Change-Id: I680edf58854d0c9be6e77c44c1e6beb1e0442a49