blst_leaf_alloc updates bighint for a leaf when an allocation is successful and includes the last block represented by the leaf. The reasoning is that, if the last block is included, then there must be no solution before that one in the leaf, so the leaf cannot provide an allocation that big again; indeed, the leaf cannot provide a solution bigger than range1.
Which is all correct, except that if the value of blk passed in did not represent the first block of the leaf, because the cursor was pointing to the middle of the leaf, then a possible solution before the cursor may have been ignored, and bighint cannot be updated.
Consider the sequence allocate 63 (returning address 0), free 0,63 (freeing that same block, and allocate 1 (returning 63. The result is that one block is allocated from the first leaf, and the value of bighint is 0, so that nothing can be allocated from that leaf until the only block allocated from that leaf is freed. This change detects that skipped-over solution, and when there is one it makes sure that the value of bighint is not changed when the last block is allocated.