Fix stack level too deep error

lib/liquid/condition.rb

@@ -7,6 +7,7 @@ module Liquid
   #   c.evaluate #=> true
   #
   class Condition #:nodoc:
+    @@depth = 0
     @@operators = {
       '=='.freeze => ->(cond, left, right) {  cond.send(:equal_variables, left, right) },
       '!='.freeze => ->(cond, left, right) { !cond.send(:equal_variables, left, right) },
@@ -47,6 +48,11 @@ module Liquid
       when :or
         result || @child_condition.evaluate(context)
       when :and
+        @@depth += 1
+        if @@depth >= 500
+          @@depth = 0
+          raise StackLevelError, "Nesting too deep".freeze
+        end
         result && @child_condition.evaluate(context)
       else
         result

test/unit/condition_unit_test.rb

@@ -130,6 +130,17 @@ class ConditionUnitTest < Minitest::Test
     assert_equal false, condition.evaluate
   end
 
+  def test_maximum_recursion_depth
+    condition = Condition.new(1, '==', 1)
+
+    assert_raises(Liquid::StackLevelError) do
+      (1..510).each do
+        condition.evaluate
+        condition.and Condition.new(2, '==', 2)
+      end
+    end
+  end
+
   def test_should_allow_custom_proc_operator
     Condition.operators['starts_with'] = proc { |cond, left, right| left =~ %r{^#{right}} }