fix: fix regression of originalIndexFromIndex

This is regression of 1969f3b
fix #4

Author: azu

src/StringSource.js

@@ -57,7 +57,8 @@ export default class StringSource {
     originalIndexFromIndex(generatedIndex) {
         let hitTokenMaps = this.tokenMaps.filter((tokenMap, index) => {
             const generated = tokenMap.generated;
-            const nextGenerated = this.tokenMaps[index + 1];
+            const nextTokenMap = this.tokenMaps[index + 1];
+            const nextGenerated = nextTokenMap ? nextTokenMap.generated : null;
             if (nextGenerated) {
                 if (generated[0] <= generatedIndex && generatedIndex < nextGenerated[0]) {
                     return true;

test/StringSource-test.js

@@ -226,14 +226,15 @@ describe("StringSource", function() {
             assert.equal(result, "alt text");
             assert.equal(source.originalIndexFromIndex(-1), null);
         });
-        it("is test for https://github.com/textlint/textlint-util-to-string/issues/4", function() {
+        it("test for https://github.com/textlint/textlint-util-to-string/issues/4", function() {
+            // related https://github.com/textlint/textlint-util-to-string/commit/1969f3b82bc490b435e2b2a631cf89a7158d80ed
             const originalText = "This link contains an [errror](index.html).";
             const AST = parse(originalText);
             const source = new StringSource(AST);
             const originalPosition = source.originalPositionFromIndex(22);
             assert.deepEqual(originalPosition, {
                 line: 1,
-                column: 24
+                column: 23
             });
         });
     });