slob: Check for NULL pointer before calling ctor()
While doing some code inspection, I noticed that the slob constructor
method can be called with a NULL pointer. If memory is tight and slob
fails to allocate with slob_alloc() or slob_new_pages() it still calls
the ctor() method with a NULL pointer. Looking at the first ctor()
method I found, I noticed that it can not handle a NULL pointer (I'm
sure others probably can't either):