os/time: fix os_localtime_r on win + add func_attr

- it makes no sense for these functions to take NULL pointers
- if `localtime()` on Windows returns a NULL pointer, the old code would try
  to dereference it.

src/nvim/os/time.c

@@ -84,7 +84,8 @@ static void microdelay(uint64_t microseconds)
 /// Portable version of POSIX localtime_r()
 ///
 /// @return NULL in case of error
-struct tm *os_localtime_r(const time_t *clock, struct tm *result)
+struct tm *os_localtime_r(const time_t *restrict clock,
+                          struct tm *restrict result) FUNC_ATTR_NONNULL_ALL
 {
 #ifdef UNIX
   // POSIX provides localtime_r() as a thread-safe version of localtime().
@@ -93,8 +94,11 @@ struct tm *os_localtime_r(const time_t *clock, struct tm *result)
   // Windows version of localtime() is thread-safe.
   // See http://msdn.microsoft.com/en-us/library/bf12f0hc%28VS.80%29.aspx
   struct tm *local_time = localtime(clock);  // NOLINT
+  if (!local_time) {
+    return NULL;
+  }
   *result = *local_time;
-return result;
+  return result;
 #endif
 }
 
@@ -103,7 +107,7 @@ return result;
 /// @param result Pointer to a 'struct tm' where the result should be placed
 /// @return A pointer to a 'struct tm' in the current time zone (the 'result'
 ///         argument) or NULL in case of error
-struct tm *os_get_localtime(struct tm *result)
+struct tm *os_get_localtime(struct tm *result) FUNC_ATTR_NONNULL_ALL
 {
   time_t rawtime = time(NULL);
   return os_localtime_r(&rawtime, result);