[BugFix] Correctly release memory of std::vector

Previously this code was plain wrong. It actually made a copy
of the vector and tried to swap its entries with the original vector.
The argument to swap even was a const reference!
Now we use the correct trick: vector().swap(other).

dune/istl/overlappingschwarz.hh

@@ -928,8 +928,7 @@ namespace Dune
   template<class I, class S, class D>
   void OverlappingSchwarzInitializer<I,S,D>::createMatrix() const
   {
-    indexMaps.clear();
-    indexMaps.swap(std::vector<IndexMap>(indexMaps));
+    std::vector<IndexMap>().swap(indexMaps);
     std::for_each(initializers->begin(), initializers->end(),
                   std::mem_fun_ref(&AtomInitializer::createMatrix));
   }