Fix #33953: blender crash after few steps with .exr image in compositor

Issue was caused by FILE multilayer ImBuf sharing buffers with render
result, and SEQUENCE multilayer ImBufs duplicating buffers. Which is
nice by it's own. But, changing image source wouldn't remove any loaded
image buffers, meaning if you've got loaded FILE multilayers they'll
likely became invalid.

That behavior of handling multilayers on changing source was done as
a fix for #24976, which is now actually not needed (removing check
for multilayer doesn't change behavior at all).

Just to be sure added check to RNA, so signal wouldn't be fired if
source wasn't actually changed form a menu.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54083 954f8c5b-7b00-dc11-b283-0030488c597c