Fix warnings for page_header_get_field() comparisons
Fix warnings for page_header_get_field() comparisons

The type uint16_t is apparently promoted to int in a comparison,
causing a sign mismatch when comparing to ulint. Convert to ulint
before comparison.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2