Don't go beyond THUNAR_N_VISIBLE_COLUMNS while parsing col widths

We can't trust last-details-view-column-widths solely as it can be changed by newer versions of thunar.

Case in point, on Ubuntu 21.04 thunar has 10 columns while on master it has 14 so if somebody tries to launch thunar from Ubuntu 21.04 after launching-quitting thunar from master then we'll be exceeding THUNAR_N_VISIBLE_COLUMNS (10 in Ubuntu 21.04 thunar).