[infer][ondemand] assert that the callbacks are set before retrieving the procedure description

Summary: This is make the callbacks are always set instead of silently returning `None`

Reviewed By: ngorogiannis

Differential Revision: D8651971

fbshipit-source-id: 7cd1c02

infer/src/backend/ondemand.ml

@@ -240,4 +240,5 @@ let clear_cache () = Typ.Procname.Hash.clear (Lazy.force cached_results)
 
 (** Find a proc desc for the procedure, perhaps loading it from disk. *)
 let get_proc_desc callee_pname =
-  match !callbacks_ref with Some callbacks -> callbacks.get_proc_desc callee_pname | None -> None
+  let callbacks = Option.value_exn !callbacks_ref in
+  callbacks.get_proc_desc callee_pname