eina: don't access dead promise.

We do properly unref promise while calling all the then callback. There
is no need to check it a second time (which actually lead to a 100%
bad access).

T3759

src/lib/eina/eina_promise.c

@@ -327,10 +327,6 @@ _eina_promise_finish(_Eina_Promise_Default_Owner* promise)
      {
         _eina_promise_then_calls(promise);
      }
-   if(promise->promise.ref == 0)
-     {
-        _eina_promise_del(promise);
-     }
 }
 
 static Eina_Error