Fix implicit pthread_exit on main thread return (closes #1767)

According to pthread_exit man page, returning from a thread function implicitly calls pthread_exit only on non-main threads.
This avoids a strange cast (with overflow check) at normal return from main.

Author: sim642

src/analyses/base.ml

@@ -2058,10 +2058,12 @@ struct
           let st' = set ~man ~t_override nst (return_var ()) t_override rv in
           match ThreadId.get_current ask with
           | `Lifted tid when ThreadReturn.is_current ask ->
-            (* Evaluate exp and cast the resulting value to the void-pointer-type.
-                Casting to the right type here avoids precision loss on joins. *)
-            let rv = VD.cast ~torg:(Cilfacade.typeOf exp) Cil.voidPtrType rv in
-            man.sideg (V.thread tid) (G.create_thread rv);
+            if not (ThreadIdDomain.Thread.is_main tid) then (
+              (* Evaluate exp and cast the resulting value to the void-pointer-type.
+                 Casting to the right type here avoids precision loss on joins. *)
+              let rv = VD.cast ~torg:(Cilfacade.typeOf exp) Cil.voidPtrType rv in
+              man.sideg (V.thread tid) (G.create_thread rv)
+            );
             Priv.thread_return ask (priv_getg man.global) (priv_sideg man.sideg) tid st'
           | _ -> st'
         ) nst exp