r53351 removed a NULL check for name in defgroup_name_index(). name can be NULL when deforming verts in the BGE, so I'm adding a NULL check back in.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53512 954f8c5b-7b00-dc11-b283-0030488c597c