`Table::create_spanned_slot_based_on_cell_above()` was performing the
subtraction `self.slots.len() - 2`, which could theoretically result
in underflow if `self.slots.len()` is 0 or 1.
That shouldn't have been possible in practice, but it may be worth
addressing, to improve code robustness. So this patch:
- Switches to `self.current_y()?.checked_sub(1)?`, which is safe and
is easier to understand.
- Moves `create_spanned_slot_based_on_cell_above()` to `TableBuilder`,
since `current_y()` is there, and the method is only used when
building the table anyways.
- Ensures that both callers use `expect()` to assert that the method
returned a value.
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
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