fix segfault when onload and onerror are not function

src/Image.cc

@@ -258,7 +258,11 @@ Image::readPNG(void *c, uint8_t *data, unsigned int len) {
 NAN_GETTER(Image::GetOnload) {
   NanScope();
   Image *img = ObjectWrap::Unwrap<Image>(args.This());
+  if (img->onload) {
     NanReturnValue(img->onload->GetFunction());
+  } else {
+    NanReturnNull();
+  }
 }
 
 /*
@@ -279,7 +283,11 @@ NAN_SETTER(Image::SetOnload) {
 NAN_GETTER(Image::GetOnerror) {
   NanScope();
   Image *img = ObjectWrap::Unwrap<Image>(args.This());
+  if (img->onerror) {
     NanReturnValue(img->onerror->GetFunction());
+  } else {
+    NanReturnNull();
+  }
 }
 
 /*

test/image.test.js

@@ -18,6 +18,8 @@ module.exports = {
     var img = new Image
       , n = 0;
 
+    assert.strictEqual(null, img.onload);
+
     assert.strictEqual(false, img.complete);
     img.onload = function(){
       ++n;
@@ -39,6 +41,8 @@ module.exports = {
       , error
       , n = 0;
 
+    assert.strictEqual(null, img.onerror);
+
     assert.strictEqual(false, img.complete);
     img.onload = function(){
       assert.fail('called onload');