Fix for crasher when node group tree datablocks are missing.

source/blender/nodes/composite/nodes/node_composite_common.c

@@ -119,6 +119,9 @@ static void *group_initexec(bNode *node)
 	bNodeSocket *sock;
 	bNodeStack *ns;
 	
+	if (!ngroup)
+		return NULL;
+	
 	/* initialize the internal node tree execution */
 	exec = ntreeCompositBeginExecTree(ngroup, 0);
 	
@@ -137,7 +140,8 @@ static void group_freeexec(bNode *UNUSED(node), void *nodedata)
 {
 	bNodeTreeExec *gexec= (bNodeTreeExec*)nodedata;
 	
-	ntreeCompositEndExecTree(gexec, 0);
+	if (gexec)
+		ntreeCompositEndExecTree(gexec, 0);
 }
 
 /* Copy inputs to the internal stack.
@@ -191,6 +195,9 @@ static void group_execute(void *data, int thread, struct bNode *node, void *node
 {
 	bNodeTreeExec *exec= (bNodeTreeExec*)nodedata;
 	
+	if (!exec)
+		return;
+	
 	/* XXX same behavior as trunk: all nodes inside group are executed.
 	 * it's stupid, but just makes it work. compo redesign will do this better.
 	 */