Check cached file size when downloading an url

In some cases cache-url fails when downloading an image and leaves
an empty cached file. qemu-img then fails with "Wrong medium type"
error on next run.

Change-Id: I23e91c52094f27248cf8452f192ad63646051190

elements/cache-url/bin/cache-url

@@ -27,7 +27,7 @@ dest=$2
 mkdir -p $(dirname $dest)
 tmp=$(mktemp $(dirname $dest)/.download.XXXXXXXX)
 
-if [ -f $dest ] ; then
+if [ -f $dest -a -s $dest ] ; then
     time_cond="-z $dest"
     success="Server copy has changed. Using server version of $url"
 else