Fix bug causing incorrect error messages

The bug could cause the list of expected strings in an error message to
contain strings that shouldn't be there.

Closes #2.

lib/compiler.js

@@ -915,6 +915,10 @@ PEG.Compiler = {
       "    },",
       "    ",
       "    _matchFailed: function(failure) {",
+      "      if (this._pos < this._rightmostMatchFailuresPos) {",
+      "        return;",
+      "      }",
+      "      ",
       "      if (this._pos > this._rightmostMatchFailuresPos) {",
       "        this._rightmostMatchFailuresPos = this._pos;",
       "        this._rightmostMatchFailuresExpected = [];",

lib/metagrammar.js

@@ -36,6 +36,10 @@ PEG.grammarParser = (function(){
     },
     
     _matchFailed: function(failure) {
+      if (this._pos < this._rightmostMatchFailuresPos) {
+        return;
+      }
+      
       if (this._pos > this._rightmostMatchFailuresPos) {
         this._rightmostMatchFailuresPos = this._pos;
         this._rightmostMatchFailuresExpected = [];