Fix a crash from crashtest

4  0x00002aaaaad1b3ab in rtl_uString_newFromSubString (ppThis=0x7ffffffeba48, pFrom=0x2aaaaad4236c <aImplEmpty_rtl_uString>, beginIndex=1, count=-2)
    at /home/julien/compile-libreoffice/libo/sal/rtl/strtmpl.cxx:1231
5  0x00002aaaec13e2b1 in rtl::OUString::copy (this=0x7ffffffebed0, beginIndex=1, count=-2) at /home/julien/compile-libreoffice/libo/include/rtl/ustring.hxx:1483
6  0x00002aaaec1717e3 in writerfilter::rtftok::RTFSdrImport::resolve (this=0x1aa4710, rShape=..., bClose=true)
    at /home/julien/compile-libreoffice/libo/writerfilter/source/rtftok/rtfsdrimport.cxx:356
7  0x00002aaaec134dfb in writerfilter::rtftok::RTFDocumentImpl::popState (this=0x20508a0)
    at /home/julien/compile-libreoffice/libo/writerfilter/source/rtftok/rtfdocumentimpl.cxx:4100
8  0x00002aaaec18327f in writerfilter::rtftok::RTFTokenizer::resolveParse (this=0x1aa46c0)
    at /home/julien/compile-libreoffice/libo/writerfilter/source/rtftok/rtftokenizer.cxx:106
9  0x00002aaaec11b269 in writerfilter::rtftok::RTFDocumentImpl::resolve (this=0x20508a0, rMapper=...)
    at /home/julien/compile-libreoffice/libo/writerfilter/source/rtftok/rtfdocumentimpl.cxx:644
10 0x00002aaaec297aa6 in RtfFilter::filter (this=0x1b658b0, aDescriptor=uno::Sequence of length 13 = {...})
    at /home/julien/compile-libreoffice/libo/writerfilter/source/filter/RtfFilter.cxx:122

Here are some debug:
8;5;(0,0);(0,13);(1694,13);(1694,0);(0,0);
aToken (0,0)
aToken (0,13)
aToken (1694,13)
aToken (1694,0)
aToken (0,0)
aToken

So must test that aToken is not empty

Change-Id: Ib5f0af3306a983c8531a5db732831d4d786ce9f5
Reviewed-on: https://gerrit.libreoffice.org/8173
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>