Fix explosion expert in rare cases

git-svn-id: http://svn.net-core.org/repos/t-engine4@5526 51575b47-30f0-44d4-a5cc-537603b46e54

game/modules/tome/data/talents/spells/explosives.lua

@@ -96,7 +96,7 @@ newTalent{
 			local theorical_nb = ({ 9, 25, 45, 77, 109, 145 })[tg.radius] or 145
 			local nb = 0
 			local grids = self:project(tg, x, y, function(tx, ty) end)
-			for px, ys in pairs(grids) do for py, _ in pairs(ys) do nb = nb + 1 end end
+			for px, ys in pairs(grids or {}) do for py, _ in pairs(ys) do nb = nb + 1 end end
 			nb = theorical_nb - nb
 			if nb > 0 then
 				local mult = math.log10(nb) / (6 - math.min(self:getTalentLevelRaw(self.T_EXPLOSION_EXPERT), 5))