fix nter flanking region

src/core/proteinmatch.cpp

@@ -818,7 +818,7 @@ ProteinMatch::getFlankingNterRegion(const PeptideMatch &peptide_match,
   int start  = peptide_match.getStart() - length;
   if(start < 0)
     {
-      length = length - start;
+      length = length + start;
       start  = 0;
     }
   if(length < 0)