[FIX] segfault for iupac assignment in dna5 stringset
[FIX] segfault for iupac assignment in dna5 stringset

bestrauc/seqan,xenigmax/seqan,xenigmax/seqan,bestrauc/seqan,ktrappe/seqan,ktrappe/seqan,ktrappe/seqan,bestrauc/seqan,bestrauc/seqan,ktrappe/seqan,ktrappe/seqan,xenigmax/seqan,bestrauc/seqan,bestrauc/seqan,bestrauc/seqan,ktrappe/seqan,ktrappe/seqan,bestrauc/seqan,xenigmax/seqan,xenigmax/seqan,ktrappe/seqan,xenigmax/seqan,xenigmax/seqan,xenigmax/seqan,ktrappe/seqan