Differing prev_entry of invalid entry behavior for in-mem vs. on-disk storage (v1.1.6 amalgamation)

[csaftoiu]

In an on-disk database, unqlite_kv_cursor_prev_entry of an invalid entry gets a valid cursor pointing to the last entry. In an on-memory database, unqlite_kv_cursor_prev_entry of an invalid entry still keeps the cursor invalid.

In a similar vein to #27, I think it should be consistent and/or explicitly documented.

[symisc]

Source Code (Amalgamation & Github) Update with the 1.1.7 release which fixes all the know bugs.

https://unqlite.org/downloads.html