pivot-table: Fix type mismatch in equal_indexes().

This function was being called with an 'unsigned int *' but it took it
as a 'size_t *', which worked fine on 32-bit systems but caused an
enormous number of tests to fail on 64-bit systems.

Thanks to Alan Mead and John Darrington for assistance.

Fixes: 5cab4cf3322f ("output: Introduce pivot tables.")