Terminate a blist_alloc search when a blst_meta_alloc calls fails with cursor == 0.
Every call to blst_meta_alloc but the one at the root is made only when the meta-node is known to include a free block, so that either the allocation will succeed, the node hint will be updated, or the last block of the meta node range is, and remains, free. But the call at the root is made without checking that there is a free block, so in the case that every block is allocated, there is no hint update to prevent the current code from looping forever.