scripts/functions: if no upstream URL is given, silently ignore that

When retrieving tarballs from upstream, if no URL was given, do not
fail; simmply ignore that fact.

This will be used later when the SVN helper will call the standard
helper to try the LAN mirror before trying svn.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""