anticipate merge_trees() returning NULL to indicate failure

Related to #1801.

diff --git a/lib/common/ns.c b/lib/common/ns.c
index 0f88c051130dc5ff8aada2160328a83006e38d7d..ec3f3031a47c204a8acb2d7ae24f586d01900653 100644 (file)
--- a/lib/common/ns.c
+++ b/lib/common/ns.c
@@ -527,6 +527,10 @@ int feasible_tree(void)
       break;
     }
     tree1 = merge_trees(ee);
+    if (tree1 == NULL) {
+      error = 2;
+      break;
+    }
     STheapify(heap,tree1->heap_index);
   }