Fix crash in when dding subsurf modifier to curves

Issue was caused by svn rev57566 which didn't take into
account the fact that curves could have dm of type CCGDM
and in this case CDDM_calc_normals_mapping_ex would fail
dramatically.

Added a check around normals update call bout whether
DM is CDDM or not. The same happens in mesh's modifier
stack.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57601 954f8c5b-7b00-dc11-b283-0030488c597c