gridview: Fix condition for adding filler tiles

Code above the condition ensures that i is always in [0, self->n_columns - 1] range, so the condition was always true, which resulted in filler tile always being added to the grid. As the result, an empty row in e.g. Nautilus appeared at the end of the grid if the number of columns divided the number of items:

Only add filler tile if last row is not full, i.e. when i > 0.