BUG: field's height can be changed unlimitedly

[poppo]

in tool_setslope_t, we check the height differences from the adjacent tiles. But the tile with field of factory, this check was skipped:

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// check, if action is valid ...
const sint16 hgt=new_pos.z;
// maximum difference check with tiles to north, south east and west
const sint8 test_hgt = hgt+(new_slope!=0);

- if(  gr1->get_typ()==grund_t::boden  ) {
+ if(  gr1->get_typ()==grund_t::boden  ||  gr1->get_typ()==grund_t::fundament  ) {
for(  sint16 i = 0 ;  i < 4 ;  i++  ) {
const koord neighbour = k + koord::nesw[i];
grund_t::fundament means there is a building or fields, but in building case, returned early.