* Electron version: 1.6.1
* Operating system: MacOS 10.12

Had a crash happen at random. Might want to glance at OnURLFetchComplete and see if there are any obvious bugs.

```
Thread 9 Crashed:: Chrome_IOThread
0   com.github.electron.framework 	0x000000010bdd3c40 0x10a237000 + 28953664
1   com.github.electron.framework 	0x000000010a2f5606 atom::URLRequestFetchJob::OnURLFetchComplete(net::URLFetcher const*) + 70
2   com.github.electron.framework 	0x000000010a2f5670 non-virtual thunk to atom::URLRequestFetchJob::OnURLFetchComplete(net::URLFetcher const*) + 16
3   com.github.electron.framework 	0x000000010a481d91 0x10a237000 + 2403729
4   com.github.electron.framework 	0x000000010a455c26 0x10a237000 + 2223142
5   com.github.electron.framework 	0x000000010a455efc 0x10a237000 + 2223868
6   com.github.electron.framework 	0x000000010a456243 0x10a237000 + 2224707
7   com.github.electron.framework 	0x000000010a458a75 0x10a237000 + 2234997
8   com.github.electron.framework 	0x000000010a45598e 0x10a237000 + 2222478
9   com.github.electron.framework 	0x000000010a46a4e3 0x10a237000 + 2307299
10  com.github.electron.framework 	0x000000010a6c6ef8 0x10a237000 + 4783864
11  com.github.electron.framework 	0x000000010a6c7086 0x10a237000 + 4784262
12  com.github.electron.framework 	0x000000010a4829f2 0x10a237000 + 2406898
13  com.github.electron.framework 	0x000000010a460577 0x10a237000 + 2266487
14  libsystem_pthread.dylib       	0x00007fffa1d55aab _pthread_body + 180
15  libsystem_pthread.dylib       	0x00007fffa1d559f7 _pthread_start + 286
16  libsystem_pthread.dylib       	0x00007fffa1d551fd thread_start + 13
```

Full dump: https://gist.github.com/pfrazee/a6fc2d2f9127892b9c4627953ebace4a
atom/browser/net/url_request_fetch_job.cc
if (!response_info_ || !response_info_->headers)
    return false;
  return response_info_->headers->GetMimeType(mime_type);
}
void URLRequestFetchJob::GetResponseInfo(net::HttpResponseInfo* info) {
  if (response_info_)
    *info = *response_info_;
}
int URLRequestFetchJob::GetResponseCode() const {
  if (!response_info_ || !response_info_->headers)
    return -1;
  return response_info_->headers->response_code();
}
void URLRequestFetchJob::OnURLFetchComplete(const net::URLFetcher* source) {
  ClearPendingBuffer();
  ClearWriteBuffer();
  if (fetcher_->GetStatus().is_success()) {
    if (!response_info_) {
      
      
      
      HeadersCompleted();
      return;
    }
    ReadRawDataComplete(0);
  } else {
    NotifyStartError(fetcher_->GetStatus());
  }
}
int URLRequestFetchJob::BufferCopy(net::IOBuffer* source, int num_bytes,