debuginfod: file:// URLs: handle curl resp. code

debuginfod: file:// URLs: handle curl resp. code

When file:// is used for DEBUGINFOD_URLS, then the response code for a
successful server query is 0 and not 200.

Using file:// can be helpful when you want to test your
debuginfod-client integration against a mocked file tree that mimics the
HTTP URLs from the debuginfod server. This way you don't have to run the
debuginfod server at all.

Fixes https://sourceware.org/bugzilla/show_bug.cgi?id=25600

Signed-off-by: Konrad Kleine <kkleine@redhat.com>