Remove some unnecessary downcasts to int.

Remove some unnecessary downcasts to int.

This value is always used as an array index so we can keep it as std::ptrdiff_t
instead of downcasting to int.

Note that the line

    if (diag_find && std::fabs(...) == 0.0 ...)

checked the wrong thing: diag_find would have, if we did not find the value, be
set to col_indices + num_entries, not NULL.