fix warning message when imported Collada nodes have < 3 edges.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54377 954f8c5b-7b00-dc11-b283-0030488c597c
fix null pointer issue when child nodes reference to unknown/invalid mesh

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54378 954f8c5b-7b00-dc11-b283-0030488c597c