Enable some asserts for impossible conditions

src/timers.c

@@ -19,11 +19,11 @@ static int timers_cmp(const RBNode *a_, const RBNode *b_)
         } else if (a->seq > b->seq) {
             return 1;
         } else { //should never happen
-            //assert(0);
+            assert(0);
             return 0;
         }
     }
-    //assert(0);
+    assert(0);
     return 0; //silence compiler
 }
 
@@ -56,8 +56,10 @@ void uc_timers_add(struct UCTimers *timers, struct UCTimer *timer, int sec, int
     struct timeval tmp;
 
     assert(timer->callback != NULL);
-    if (timer->callback == NULL) //must be set. 
+    if (timer->callback == NULL) { //must be set. 
+        assert(timer->callback != NULL);
         return;
+    }
 
     tmp.tv_sec = sec;
     tmp.tv_usec = usec;