rtl: fpwidestring - fix length of UnicodeStrings after UTF8ToUnicode. UTF8ToUnicode returns length + 1 for zero byte which is not needed for UnicodeString.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23800 3ad0048d-3df7-0310-abae-a5850022a9f2