qom: Fix class cast of NULL classes

qom: Fix class cast of NULL classes

Its clear from the implementation that class casting is supposed to work
with a NULL class argument. Guard all dereferences of the class argument
against NULL accordingly.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 94cd5ba46b74eea289a7e582635820c1c54e66fa.1371546907.git.peter.crosthwaite@xilinx.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 9d6a3d58e4d1431ab3809ff621cfd1f9ec75eef5)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>