Allow mossy cobble slabs to combine (#1791)
That was nil for a reason 2199be5 #1118 #1120
@paramat This PR is just an extension for PR 1120, as the players seem to want to craft them. There's nothing wrong with crafting mossy stairs or slabs from entire nodes and backwards.
Should we remove the code which copes with 'nil' too?
I guess that one can be left this way, if a modder really wants to add such special stairs or slabs (they'll always find an use for it).