Use a loop to evaluate binary comparisions to avoid recursion (#891)

Using recursion allows a malicious template to cause a SystemStackError

lib/liquid/condition.rb

@@ -41,16 +41,22 @@ module Liquid
     end
 
     def evaluate(context = Context.new)
-      result = interpret_condition(left, right, operator, context)
+      condition = self
+      result = nil
+      loop do
+        result = interpret_condition(condition.left, condition.right, condition.operator, context)
 
-      case @child_relation
+        case condition.child_relation
         when :or
-        result || @child_condition.evaluate(context)
+          break if result
         when :and
-        result && @child_condition.evaluate(context)
+          break unless result
         else
-        result
+          break
         end
+        condition = condition.child_condition
+      end
+      result
     end
 
     def or(condition)
@@ -75,6 +81,10 @@ module Liquid
       "#<Condition #{[@left, @operator, @right].compact.join(' '.freeze)}>"
     end
 
+    protected
+
+    attr_reader :child_relation, :child_condition
+
     private
 
     def equal_variables(left, right)