Fix #31272: Blender Crashes when press render

Issue was caused by missed libraries. Seems compositor nodes already does check
in such cases, added the same check to texture and shader nodes.

source/blender/nodes/shader/nodes/node_shader_common.c

@@ -73,6 +73,9 @@ static void *group_initexec(bNode *node)
 	bNodeTree *ngroup= (bNodeTree*)node->id;
 	bNodeTreeExec *exec;
 	
+	if (!ngroup)
+		return NULL;
+	
 	/* initialize the internal node tree execution */
 	exec = ntreeShaderBeginExecTree(ngroup, 0);
 	

source/blender/nodes/texture/nodes/node_texture_common.c

@@ -58,6 +58,9 @@ static void *group_initexec(bNode *node)
 	bNodeTree *ngroup= (bNodeTree*)node->id;
 	void *exec;
 	
+	if (!ngroup)
+		return NULL;
+	
 	/* initialize the internal node tree execution */
 	exec = ntreeTexBeginExecTree(ngroup, 0);