algo/core/germline.cpp: Count index reference just once

If we try to set the same index several times we will wrongly have several references to it and, thus,
will never be deleted, which Valgrind dislikes

See !674

algo/core/germline.cpp

@@ -182,8 +182,10 @@ void Germline::new_index(IndexTypes type)
 
 void Germline::set_index(IKmerStore<KmerAffect> *_index)
 {
-  index = _index;
-  index->refs++ ;
+  if (index != _index) {
+    index = _index;
+    index->refs++ ;
+  }
 }