Fix ticket 337: Value elements may not have a params field.

diff --git a/OvaleFrame.lua b/OvaleFrame.lua
index 17cf5a7..b0d2866 100644
--- a/OvaleFrame.lua
+++ b/OvaleFrame.lua
@@ -1,7 +1,7 @@
 --[[--------------------------------------------------------------------
     Ovale Spell Priority
     Copyright (C) 2009, 2010, 2011, 2012 Sidoine
-    Copyright (C) 2012, 2013 Johnny C. Lam
+    Copyright (C) 2012, 2013, 2014 Johnny C. Lam

     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License in the LICENSE
@@ -202,7 +202,7 @@ do
 				end
 				if element and element.type == "value" then
 					local actionTexture
-					if node.params.texture then
+					if node.params and node.params.texture then
 						actionTexture = API_GetSpellTexture(node.params.texture)
 					end
 					local value