Fail early when processing (test) nodes.

If a certain segment yields no nodes the on_path() method should not process any
following segments.

lib/oga/xpath/evaluator.rb

@@ -61,6 +61,8 @@ module Oga
 
           if test != last_node and !@stack.empty?
             swap_context
+          elsif @stack.empty?
+            break
           end
         end
       end

spec/oga/xpath/evaluator/paths_spec.rb

@@ -24,6 +24,16 @@ describe Oga::XPath::Evaluator do
     end
   end
 
+  context 'invalid absolute paths' do
+    before do
+      @set = @evaluator.evaluate('/x/a')
+    end
+
+    example 'return an empty NodeSet' do
+      @set.empty?.should == true
+    end
+  end
+
   context 'relative paths' do
     before do
       @set = @evaluator.evaluate('a')
@@ -42,6 +52,16 @@ describe Oga::XPath::Evaluator do
     end
   end
 
+  context 'invalid relative paths' do
+    before do
+      @set = @evaluator.evaluate('x/a')
+    end
+
+    example 'return an empty NodeSet' do
+      @set.empty?.should == true
+    end
+  end
+
   context 'nested paths' do
     before do
       @set = @evaluator.evaluate('/a/b')