drive: Fix a crash in gdata::DriveUploader::ResumeUpload()

drive: Fix a crash in gdata::DriveUploader::ResumeUpload()

This is a reland of crrev.com/163511, which was reverted as "return"
statement was missing where it should not.

The crash reported at crbug.com/157057 can occur if the 2nd parameter to
std::string constructor is negative here:

bool ResumeUploadOperation::GetContentData(std::string* upload_content_type,
                                           std::string* upload_content) {
  *upload_content_type = params_.content_type;
  *upload_content = std::string(params_.buf->data(),
  params_.end_range - params_.start_range + 1);
  return true;
}

Looking at the code, this can occur if reading of a file failed in
DriveUploader::ReadCompletionCallback().

BUG=157057
TEST=none. This crash is hard to reproduce

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=163745

[kinaba, 2012-10-23 07:11:53]

lgtm