Fix SIGABRT due to invalid heap access

When the set update completed with "inconsistent" state, the heap is in
an invalid state and should not be accessed.

In particular, consider the case of `SAMP -> AGG_L1 -> ldms_ls`. SAMP
has a sampler that uses `LDMS_V_LIST`. AGG_L1 has finished looking up
the set from SAMP, and is in the middle of updating it. At this point,
the metadata on AGG_L1 has been populated, but the data+heap parts are
still uninitialized. In the mean time, `ldms_ls` lookup + update the set
and get SIGABRT from `ldms_heap_get()` due to bad heap signature because
of the uninitialized data+heap part on the AGG_L1 that `ldms_ls` copied
the data from.

This patch:
- adds set consistency check before getting the heap in the LDMS set
  update path.
- adds heap pointer check in the `ldms_list_first()` list/heap access.

ldms/src/core/ldms.c

@@ -2642,6 +2642,8 @@ ldms_mval_t ldms_list_first(ldms_set_t s, ldms_mval_t lh, enum ldms_value_type *
 	ldms_mval_t le;
 	if (!lh->v_lh.head)
 		return NULL;
+	if (!s->heap)
+		return NULL;
 	le = ldms_heap_ptr(s->heap, lh->v_lh.head);
 	if (typ)
 		*typ = le->v_le.type;

ldms/src/core/ldms_xprt.c

@@ -2273,7 +2273,10 @@ static void __handle_update_data(ldms_t x, struct ldms_context *ctxt,
 		}
 		if (set->meta->heap_sz) {
 			base = ((void*)set->data) + set->data->size - set->meta->heap_sz;
-			set->heap = ldms_heap_get(&set->heap_inst, &set->data->heap, base);
+			if (ldms_set_is_consistent(set))
+				set->heap = ldms_heap_get(&set->heap_inst, &set->data->heap, base);
+			else
+				set->heap = NULL;
 		}
 		ctxt->update.cb(x, set, flags, ctxt->update.cb_arg);
 		prev_data = data;