sde_lib: Allow group placeholders.

If reading a group has been requested from the application/tool layer
(though PAPI_event_name_to_code()) before the group is actually registered
by the library, we will create a placeholder for it. This change allows
the group registration to overwrite the placeholder.

src/sde_lib/sde_lib.c

@@ -519,7 +519,12 @@ papi_sde_add_counter_to_group(papi_handle_t handle, const char *event_name, cons
 
     }else{
         if( NULL == tmp_group->u.cntr_group.group_head ){
-            SDE_ERROR("papi_sde_add_counter_to_group(): Found an empty counter group: '%s'. This might indicate that a cleanup routine is not doing its job.", group_name);
+            if( CNTR_CLASS_PLACEHOLDER == tmp_group->cntr_class ){
+                tmp_group->cntr_class = CNTR_CLASS_GROUP;
+            }else{
+                SDE_ERROR("papi_sde_add_counter_to_group(): Found an empty counter group: '%s'. This might indicate that a cleanup routine is not doing its job.", group_name);
+            }
+
         }
 
         // make sure the caller is not trying to change the flags of the group after it has been created.