In Player:MarkExclusions(): Don't check the recipe's KNOWN state until it has been determined that recipe is not nil. Fixes ticket #987.

diff --git a/Player.lua b/Player.lua
index 6eeb0a8..84121de 100644
--- a/Player.lua
+++ b/Player.lua
@@ -76,10 +76,9 @@ function Player:MarkExclusions()

 	for spell_id in pairs(exclusion_list) do
 		local recipe = recipe_list[spell_id]
-		local is_known = recipe:HasState("KNOWN")

 		if recipe then
-			if is_known and recipe.profession == profession then
+			if recipe:HasState("KNOWN") and recipe.profession == profession then
 				known_count = known_count + 1
 			elseif recipe_profession == profession then
 				unknown_count = unknown_count + 1