unix, windows: don't allow a NULL callback on timers

src/unix/timer.c

@@ -65,6 +65,9 @@ int uv_timer_start(uv_timer_t* handle,
                    uint64_t repeat) {
   uint64_t clamped_timeout;
 
+  if (cb == NULL)
+    return -EINVAL;
+
   if (uv__is_active(handle))
     uv_timer_stop(handle);
 

src/win/timer.c

@@ -119,6 +119,9 @@ int uv_timer_start(uv_timer_t* handle, uv_timer_cb timer_cb, uint64_t timeout,
   uv_loop_t* loop = handle->loop;
   uv_timer_t* old;
 
+  if (timer_cb == NULL)
+    return UV_EINVAL;
+
   if (handle->flags & UV_HANDLE_ACTIVE) {
     RB_REMOVE(uv_timer_tree_s, &loop->timers, handle);
   }

test/test-list.h

@@ -133,6 +133,7 @@ TEST_DECLARE   (timer_huge_timeout)
 TEST_DECLARE   (timer_huge_repeat)
 TEST_DECLARE   (timer_run_once)
 TEST_DECLARE   (timer_from_check)
+TEST_DECLARE   (timer_null_callback)
 TEST_DECLARE   (idle_starvation)
 TEST_DECLARE   (loop_handles)
 TEST_DECLARE   (get_loadavg)
@@ -451,6 +452,7 @@ TASK_LIST_START
   TEST_ENTRY  (timer_huge_repeat)
   TEST_ENTRY  (timer_run_once)
   TEST_ENTRY  (timer_from_check)
+  TEST_ENTRY  (timer_null_callback)
 
   TEST_ENTRY  (idle_starvation)
 

test/test-timer.c

@@ -290,3 +290,14 @@ TEST_IMPL(timer_run_once) {
   MAKE_VALGRIND_HAPPY();
   return 0;
 }
+
+
+TEST_IMPL(timer_null_callback) {
+  uv_timer_t handle;
+
+  ASSERT(0 == uv_timer_init(uv_default_loop(), &handle));
+  ASSERT(UV_EINVAL == uv_timer_start(&handle, NULL, 100, 100));
+
+  MAKE_VALGRIND_HAPPY();
+  return 0;
+}