Ensure that the devices array exists also when not using MAGMA

src/cluster/cluster.cpp

@@ -110,6 +110,9 @@ void cluster(const string& config_file, const string& data_file, const string& o
     delete logger;
     return;
   }
+#else
+  int *devices = new int[1];
+  devices[0] = -1;
 #endif
   // the following only happens on MPI process 0
   if (mpidata->rank == 0) {