qom/object: Don't poll cast cache for NULL objects

object_dynamic_cast_assert used to be tolerant of NULL objects and not
assert. It's clear from the implementation that this is the expected
behavior.

The preceding check of the cast cache dereferences obj however causing
a segfault. Fix by conditionalizing the cast cache logic on obj being
non-null.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Message-id: 8e2bef6a55753869c50bfa32226f7fcf0439ca62.1369183592.git.peter.crosthwaite@xilinx.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
