Reset parser position when action returns |null|

The change does not change the benchmark suite execution speed
statistically significantly.

Detailed results (benchmark suite totals as reported by "jake benchmark"
on Node.js 0.4.8):

-----------------------------------
 Test #      Before        After
-----------------------------------
      1   128.20 kB/s   128.03 kB/s
      2   130.36 kB/s   128.73 kB/s
      3   126.53 kB/s   129.72 kB/s
      4   127.46 kB/s   127.48 kB/s
      5   127.63 kB/s   128.53 kB/s
-----------------------------------
Average   128.04 kB/s   125.50 kB/s
-----------------------------------

Closes GH-25.

src/emitter.js

@@ -627,6 +627,8 @@ PEG.compiler.emitter = function(ast) {
        */
 
       var expressionResultVar = UID.next("result");
+      var actionResultVar     = UID.next("result");
+      var savedPosVar         = UID.next("savedPos");
 
       if (node.expression.type === "sequence") {
         var formalParams = [];
@@ -649,16 +651,25 @@ PEG.compiler.emitter = function(ast) {
       }
 
       return formatCode(
+        "var ${savedPosVar} = pos;",
         "${expressionCode}",
-        "var ${resultVar} = ${expressionResultVar} !== null",
+        "var ${actionResultVar} = ${expressionResultVar} !== null",
         "  ? (function(${formalParams}) {${actionCode}})(${actualParams})",
         "  : null;",
+        "if (${actionResultVar} !== null) {",
+        "  var ${resultVar} = ${actionResultVar};",
+        "} else {",
+        "  var ${resultVar} = null;",
+        "  pos = ${savedPosVar};",
+        "}",
         {
           expressionCode:      emit(node.expression, expressionResultVar),
           expressionResultVar: expressionResultVar,
           actionCode:          node.code,
+          actionResultVar:     actionResultVar,
           formalParams:        formalParams.join(", "),
           actualParams:        actualParams.join(", "),
+          savedPosVar:         savedPosVar,
           resultVar:           resultVar
         }
       );

test/compiler-test.js

@@ -129,6 +129,13 @@ test("actions", function() {
   );
   parses(innerElementsLabeledParser, "abcde", ["a", [2, "b", "d"], "e"]);
 
+  /*
+   * Test that the parsing position returns after successfull parsing of the
+   * action expression and action returning |null|.
+   */
+  var posTestParser = PEG.buildParser('start = "a" { return null; } / "a"');
+  parses(posTestParser, "a", "a");
+
   /* Test that the action is not called when its expression does not match. */
   var notAMatchParser = PEG.buildParser(
     'start = "a" { ok(false, "action got called when it should not be"); }'