stop progress callbacks after curl_easy_perform returns

This prevents possible null dereferences in FTP transfers when the
progress callback is touched during connection teardown.

http://curl.haxx.se/mail/lib-2011-08/0128.html

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/dload.c

@@ -302,6 +302,9 @@ static int curl_download_internal(struct dload_payload *payload,
 	/* perform transfer */
 	handle->curlerr = curl_easy_perform(handle->curl);
 
+	/* immediately unhook the progress callback */
+	curl_easy_setopt(handle->curl, CURLOPT_NOPROGRESS, 1L);
+
 	/* was it a success? */
 	if(handle->curlerr == CURLE_ABORTED_BY_CALLBACK) {
 		goto cleanup;