assert in kasan_set_poison() if `size` is zero

e.g. mm_realloc() => kasan_poison() => kasan_set_poison()
  _assert: Assertion failed p != ((void*)0): at file: kasan/kasan.c:239 task: nsh_main process: nsh_main 0x4402caa5
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

mm/kasan/kasan.c

@@ -233,6 +233,11 @@ static void kasan_set_poison(FAR const void *addr, size_t size,
   uintptr_t mask;
   int flags;
 
+  if (size == 0)
+    {
+      return;
+    }
+
   flags = spin_lock_irqsave(&g_lock);
 
   p = kasan_mem_to_shadow(addr, size, &bit);