Fix two crash introducted by #12955
When a quicklist node can't be inserted and split, we eventually merge
the current node with its neighboring
nodes after inserting, and compress the current node and its siblings.
1. When the current node is merged with another node, the current node
may become invalid and can no longer be used.
Solution: let `_quicklistMergeNodes()` return the merged nodes.
3. If the current node is a LZF quicklist node, its recompress will be
1. If the split node can be merged with a sibling node to become head or
tail, recompress may cause the head and tail to be compressed, which is
not allowed.
Solution: always recompress to 0 after merging.
Signed-off-by: Ping Xie <pingxie@google.com>
01473fbd5c