ensure no NULL is passed to memcpy

NULL is valid when size is 0, but memcpy uses nonnull attributes,
so let's not poke the bear

contrib/epee/src/buffer.cpp

@@ -64,7 +64,8 @@ void buffer::append(const void *data, size_t sz)
       size_t reserve = (((size() + sz) * 3 / 2) + 4095) & ~4095;
       new_storage.reserve(reserve);
       new_storage.resize(size());
-      memcpy(new_storage.data(), storage.data() + offset, storage.size() - offset);
+      if (size() > 0)
+        memcpy(new_storage.data(), storage.data() + offset, storage.size() - offset);
       offset = 0;
       std::swap(storage, new_storage);
     }