Fix "pacman -U <url>" operations

Commit e6a6d307 detected complete part files by comparing a payload's
max_size to initial_size.  However, these values are also equal when we
use pacman -U on a URL as max_size is set to 0 in that case.  Add a further
condition to avoid that.

Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/dload.c

@@ -415,7 +415,7 @@ static int curl_download_internal(struct dload_payload *payload,
 
 	curl_set_handle_opts(payload, curl, error_buffer);
 
-	if(payload->max_size == payload->initial_size) {
+	if(payload->max_size == payload->initial_size && payload->max_size != 0) {
 		/* .part file is complete */
 		ret = 0;
 		goto cleanup;