Fix #34359: 2.66 crashes when using output node

16bit PNG and Jpeg2K exporters were not aware of the simple
fact that float buffer could have only 3 or 1 channels.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54746 954f8c5b-7b00-dc11-b283-0030488c597c