recovery: check return value of `fiber_new_system` for watcher fiber

`fiber_new_system` can potentially fail — its return value for the watcher
fiber must be checked and an exception must be raised in case it does fail.

Closes tarantool/security#87

NO_CHANGELOG=<security fix>
NO_DOC=<security fix>
NO_TEST=<no test harness for checking OOM>

src/box/recovery.cc

@@ -528,6 +528,8 @@ recovery_follow_local(struct recovery *r, struct xstream *stream,
 	 */
 	assert(r->watcher == NULL);
 	r->watcher = fiber_new_system(name, hot_standby_f);
+	if (r->watcher == NULL)
+		diag_raise();
 	fiber_set_joinable(r->watcher, true);
 	fiber_start(r->watcher, r, stream, wal_dir_rescan_delay);
 }