Fix #35599: MovieClip node crashes when using multilayer exr

Multilayer EXR is not supported as a source for movie clip yet,
but there's no excuse to crash!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57202 954f8c5b-7b00-dc11-b283-0030488c597c