Proposed fix for T53263 -- Blender crashes when rendering with Stabilizer 2D node without movie selected

The program won't crash anymore, but a warning won't be displayed. Anyway, this gives the user the chance to save the project.

https://developer.blender.org/T53263

Reviewers: lukastoenne

Differential Revision: https://developer.blender.org/D2934

source/blender/compositor/operations/COM_MovieClipAttributeOperation.cpp

@@ -35,6 +35,9 @@ MovieClipAttributeOperation::MovieClipAttributeOperation() : NodeOperation()
 
 void MovieClipAttributeOperation::initExecution()
 {
+	if (this->m_clip == NULL) {
+		return;
+	}
 	float loc[2], scale, angle;
 	loc[0] = 0.0f;
 	loc[1] = 0.0f;