topo/base: fix uninitialized variable

this commit fixes a bug introduced by commit open-mpi/ompi@e7c59e3adb

ompi/mca/topo/base/topo_base_dist_graph_create.c

@@ -292,11 +292,11 @@ int mca_topo_base_dist_graph_create(mca_topo_base_module_t* module,
     ompi_communicator_t *new_comm;
     mca_topo_base_comm_dist_graph_2_2_0_t* topo;
 
+    num_procs = ompi_comm_size(comm_old);
     topo_procs = (ompi_proc_t**)malloc(num_procs * sizeof(ompi_proc_t *));
     if (NULL == topo_procs) {
         return OMPI_ERR_OUT_OF_RESOURCE;
     }
-    num_procs = ompi_comm_size(comm_old);
     new_comm = ompi_comm_allocate(num_procs, 0);
     if (NULL == new_comm) {
         free(topo_procs);