addition: destroy graph

include/graph.h

@@ -10,7 +10,8 @@ typedef struct Edge Edge;
 typedef enum {
     GRAPH_OK,
     GRAPH_BAD_ALLOC,
-    GRAPH_NOT_FOUND,
+    GRAPH_VERTEX_NOT_FOUND,
+    GRAPH_NULL_ARG,
 } GraphErr ;
 
 typedef enum {
@@ -19,7 +20,7 @@ typedef enum {
 } GraphType;
 
 Graph *graph_create(GraphType type);
-GraphErr graph_destroy(Graph *graph);
+GraphErr graph_destroy(Graph **graph);
 
 GraphErr graph_add_vertex(Graph *graph);