rest_client: Fix header list management with async requests

The list of additional headers must not be freed too early, as libcurl
may still make use of it during each async resume callback.

Credits for reporting and helping with troubleshooting and testing
go to Agalya Ramachandran.

Fixes #1072

(cherry picked from commit 29363ca)

Conflicts:
	modules/rest_client/rest_methods.c

(cherry picked from commit 4ef4885)

Conflicts:
	modules/rest_client/rest_methods.c

modules/rest_client/rest_methods.c

@@ -319,7 +319,7 @@ int start_async_http_req(struct sip_msg *msg, enum rest_client_method method,
 	goto error;
 
 success:
-	clean_header_list(list);
+	async_parm->header_list = list;
 	async_parm->handle = handle;
 	async_parm->multi_list = multi_list;
 	return fd;
@@ -388,6 +388,8 @@ enum async_ret_code resume_async_http_req(int fd, struct sip_msg *msg, void *_pa
 		return 1;
 	}
 
+	curl_slist_free_all(param->header_list);
+
 	if (del_transfer(fd) != 0) {
 		LM_BUG("failed to delete fd %d", fd);
 		abort();

modules/rest_client/rest_methods.h

@@ -57,6 +57,7 @@ typedef struct rest_async_param_ {
 	enum rest_client_method method;
 	OSS_CURLM *multi_list;
 	CURL *handle;
+	struct curl_slist *header_list;
 	str body;
 	str ctype;