* src/intervals.c: Tighten assertions.
(create_root_interval): Make sure the interval is not empty.
(intervals_equal): Use booleans.
(rotate_right, rotate_left): Check LENGTHs rather than TOTAL_LENGTH.
(balance_an_interval): Sanity check LENGTHs and TOTAL_LENGTHs.
(balance_possible_root_interval): Simplify and use booleans.
(split_interval_right, split_interval_left): Check LENGTH, and remove
now redundant assertion.
(adjust_intervals_for_insertion): Remove now redundant assertions.
(delete_node, interval_deletion_adjustment)
(adjust_intervals_for_deletion, merge_interval_right)
(merge_interval_left): Check LENGTH rather than TOTAL_LENGTH.
(reproduce_interval): Make sure the interval is not empty.