Return null if no charset matches

Fixes a segmentation fault if no charset matches. According to http://icu-project.org/apiref/icu4c/ucsdet_8h.html#aff2633b5055d472cff4108d94f97cf7d, ucsdet_detect() may return NULL if no charset matches.

Fixes: #15
Co-authored-by: chenzhip <letme2010@gmail.com>

README.md

@@ -27,6 +27,8 @@ console.log(charsetMatch);
 // }
 ```
 
+detect-character-encoding may return `null` if no charset matches.
+
 ## Supported operating systems
 
 - macOS High Sierra

icuWrapper.cpp

@@ -40,6 +40,12 @@ NAN_METHOD(DetectCharacterEncoding) {
  return;
  }
 
+ if(charsetMatch == NULL) {
+ info.GetReturnValue().Set(Nan::Null());
+ ucsdet_close(charsetDetector);
+ return;
+ }
+
  const char *charsetName = ucsdet_getName(charsetMatch, &errorCode);
 
  if(U_FAILURE(errorCode)) {

test/index.js

@@ -18,6 +18,10 @@ it('should return a confidence value', () => {
  assert(typeof detectCharacterEncoding(getFixture('utf-8.txt')).confidence === 'number');
 });
 
+it('should return null if no charset matches', () => {
+ assert.strictEqual(detectCharacterEncoding(Buffer.from([0xAB])), null);
+});
+
 it('should throw a TypeError if argument is not a buffer', () => {
  assert.throws(() => {
  detectCharacterEncoding('string');