Ignore empty files returned by curl

In cases where servers ignore the Modified time, curl cancels the
download, outputs a http 200 and leaves the output file untouched, we
don't want this empty file.

Fixes bug #1234926

Change-Id: I05b0dd95dcd53ca50d88ec07f2f1ee9958b6adb7
Merge "Ignore empty files returned by curl"