Fix crash on TNT explosions if tnt is disabled

mods/tnt/init.lua

@@ -292,10 +292,15 @@ local function tnt_explode(pos, radius, ignore_protection, ignore_on_blast, owne
 	local a = VoxelArea:new({MinEdge = minp, MaxEdge = maxp})
 	local data = vm1:get_data()
 	local count = 0
-	local c_tnt = minetest.get_content_id("tnt:tnt")
+	local c_tnt
 	local c_tnt_burning = minetest.get_content_id("tnt:tnt_burning")
 	local c_tnt_boom = minetest.get_content_id("tnt:boom")
 	local c_air = minetest.get_content_id("air")
+	if enable_tnt then
+		c_tnt = minetest.get_content_id("tnt:tnt")
+	else
+		c_tnt = c_tnt_burning -- tnt is not registered if disabled
+	end
 	-- make sure we still have explosion even when centre node isnt tnt related
 	if explode_center then
 		count = 1