fixes this bt:
3  0x00002aaaab314ef2 in __GI___assert_fail (assertion=assertion@entry=0x2aaaaf8dc448 "index >= 0 && static_cast<sal_uInt32>(index) < static_cast<sal_uInt32>(getLength())",
    file=file@entry=0x2aaaaf8ac488 "/home/julien/lo/libreoffice/include/rtl/ustring.hxx", line=line@entry=678,
    function=function@entry=0x2aaaaf8dc800 <rtl::OUString::operator[](int) const::__PRETTY_FUNCTION__> "sal_Unicode rtl::OUString::operator[](sal_Int32) const")
    at assert.c:101
4  0x00002aaaaf810383 in rtl::OUString::operator[] (this=this@entry=0x55555d39c9f8, index=index@entry=0) at /home/julien/lo/libreoffice/include/rtl/ustring.hxx:678
5  0x00002aaaaf822f4e in SbiScanner::NextSym (this=0x55555d39c9d0) at /home/julien/lo/libreoffice/basic/source/comp/scanner.cxx:572
6  0x00002aaaaf82771e in SbiTokenizer::Next (this=this@entry=0x55555d39c9d0) at /home/julien/lo/libreoffice/basic/source/comp/token.cxx:339
7  0x00002aaaaf827d23 in SbiTokenizer::Peek (this=0x55555d39c9d0) at /home/julien/lo/libreoffice/basic/source/comp/token.cxx:266
8  0x00002aaaaf8175d0 in SbiExpression::Unary (this=this@entry=0x55555d43cea0) at /home/julien/lo/libreoffice/basic/source/comp/exprtree.cxx:581
9  0x00002aaaaf8178a4 in SbiExpression::Exp (this=this@entry=0x55555d43cea0) at /home/julien/lo/libreoffice/basic/source/comp/exprtree.cxx:633
10 0x00002aaaaf81792c in SbiExpression::MulDiv (this=this@entry=0x55555d43cea0) at /home/julien/lo/libreoffice/basic/source/comp/exprtree.cxx:647
11 0x00002aaaaf8179b6 in SbiExpression::IntDiv (this=this@entry=0x55555d43cea0) at /home/julien/lo/libreoffice/basic/source/comp/exprtree.cxx:666

Change-Id: Ib56063b3106bd077cda215e34ab763bae1f33c33
Reviewed-on: https://gerrit.libreoffice.org/33933Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Julien Nabet <serval2412@yahoo.fr>