Revert "Avoid NULL check in free() and malloc_usable_size()."

This reverts commit 96d4120a.

ivsalloc() depends on chunks_rtree being initialized.  This can be
worked around via a NULL pointer check.  However,
thread_allocated_tsd_get() also depends on initialization having
occurred, and there is no way to guard its call in free() that is
cheaper than checking whether ptr is NULL.