Dereference of potentially null field

If the field is actually null, then a null pointer dereference will occur. Field, which is statistically checked against null, is dereferenced.
/src/block/block_compact.c:21: NULL_FIELD 183865 Reading field "session", which is expected to possibly be "NULL" in "((WT_CONNECTION_IMPL *)((WT_SESSION_IMPL *)session)>iface.connection)>background_compact.session" (checked 1 out of 3 times).
/src/block/block_compact.c:21: NULL_FIELD 183865 Dereferencing "((WT_CONNECTION_IMPL *)((WT_SESSION_IMPL *)session)>iface.connection)>background_compact.session", which is known to be "NULL".