blockchain_converter: delete blockchain on succesful exit

While the dtor implementation does not actually do anything, other
paths do delete it, and the dtor might do someting later.

src/blockchain_converter/blockchain_converter.cpp

@@ -99,5 +99,6 @@ int main(int argc, char* argv[])
     }
   }
 
+  delete blockchain;
   return 0;
 }