Card.selectable_from_pack null check (#997)

I'm not sure exactly what error I had gotten that this fixes (It may have been a voucher pack from the Betmma Mods suite?), but this does fix it.

review comment: Actually I think this check is needed now for mod compat: probably selectable_from_pack was called by means other than smods opening a pack under smods' control

Author: blamperer

src/utils.lua

@@ -2310,12 +2310,12 @@ G.FUNCS.can_select_from_booster = function(e)
 
 function Card.selectable_from_pack(card, pack)
     if card.config.center.select_card then return card.config.center.select_card end
-    if pack.select_exclusions then
+    if pack and pack.select_exclusions then
         for _, key in ipairs(pack.select_exclusions) do
             if key == card.config.center_key then return false end
         end
     end
-    if pack.select_card then
+    if pack and pack.select_card then
         if type(pack.select_card) == 'table' then
             if pack.select_card[card.ability.set] then return pack.select_card[card.ability.set] else return false end
         end