handle locking more carefully (especially protect gc and stack shifting).

C/inlines.c

@@ -432,9 +432,11 @@ p_dif( USES_REGS1 )
 #ifdef COROUTINING
     /* now restore Woken Goals to its old value */
     Yap_UpdateTimedVar(LOCAL_WokenGoals, OldWokenGoals);
+    LOCK(LOCAL_SignalLock);
     if (OldWokenGoals == TermNil) {
       Yap_undo_signal(YAP_WAKEUP_SIGNAL);
     }
+    UNLOCK(LOCAL_SignalLock);
 #endif
     /* restore B */
     B = pt1;