Not all rows returned by the C API

We noticed after upgrading from MySQL 5.5 to MariaDB 10.1 that looping over a table through the C API doesn't return all rows. It only happens on a sufficiently large table.

Attached is a small Python script which produces such a table, and a C program that triggers the bug. Running make will compile the C file and produce a file setup.sql which you can import in your database. When it is imported, MYSQLPASSWORD= ./main -s (other flags might be needed, see ./main -h) shows the number of rows received, which should of course agree with SELECT COUNT(*) FROM foo; from the mysql client.

We have bisected the issue to this commit: https://github.com/MariaDB/server/commit/7a387c01586ae5107f2c2c64d3cdd420de83e168