Do not complain about existing empty files

Sometimes we pre-create the target filename using mktemp. In that case of
course the hash will not match, so avoid comparing.

Change-Id: Id0feb0178d659e03f5ceb000f738167cdc28a3ec
Reviewed-by: Heikki Halmet <heikki.halmet@qt.io>

coin/provisioning/common/unix/DownloadURL.sh

@@ -100,10 +100,18 @@ DownloadURL () {
         targetFile=$4
     fi
 
+    # If a non-empty file already exists
+    if [ -s "$targetFile" ]
+    then
         if   VerifyHash "$targetFile" "$expectedHash"
         then
             echo "Skipping download, found and validated existing file:  $targetFile"
+            return
         else
+            echo "WARNING: Non-empty but different file found at destination; will re-download and overwrite file:  $targetFile"
+        fi
+    fi
+
     echo "Downloading from primary URL:  $url"
     if  ! Download "$url" "$targetFile"
     then
@@ -115,5 +123,4 @@ DownloadURL () {
         fi
     fi
     VerifyHash "$targetFile" "$expectedHash"
-    fi
 }