refactor: use a more idiomatic loop to iterate over the cells

There are two cases in which the following assertion would fail:
```c
assert(g->icell < g->ncells);
```

1. If `g->ncells = 0`. Update this to be legal.
2. If an EOF is reached while parsing `wrap`. In this case, the unpacker
   attempts to resume from `cells`, which is a bug. Create a new state
   for parsing `wrap`.

Reference: https://neovim.io/doc/user/ui.html#ui-event-grid_line