Fix segfaults in the case of SVR, OneClass.

Some fields of SVM are NULL in the case of SRV & OneClass, so accesing
them resulted in segfaults.

Also add some tests. Some more tests are needed, though.

From: Fabian Pedregosa <fabian.pedregosa@inria.fr>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@548 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8