Occasionally, the test innodb_gis.rtree_recovery,4k runs out of
buffer pool during crash recovery.

This test might have started failing in connection with MDEV-18726.
Work around it by reserving a little more overhead for the recovery
hash tables.

FIXME: Propagate the "out of memory" error to recv_add_to_hash_table()
and transition to multi-batch recovery with a finer granularity.