用在Python程序中，Python运行结束时析构独显设备的VulkanDevice时释放dummy_image失败引发SIGSEGV #2666
Description
问题描述：
waifu2x-ncnn-vulkan-python（封装了waifu2x-ncnn-vulkan所以使用了ncnn）的样例程序在运行结束时，Waifu2x对象析构成功后，析构ncnn::g_default_vkdev的dummy_image的时候会发生Segment fault。在核显设备上（i5 1035G7 Iris Plus）不会有问题，但是在另一台独显设备上（1050Ti）会发生。(两台设备均为单GPU，也就是单核显和单独显)。另外，运行原版waifu2x-ncnn-vulkan程序的时候都没有问题。系统均为Arch linux
复现步骤：
1.编译waifu2x-ncnn-vulkan-python
2.到编译目录中运行waifu2x_ncnn_vulkan.py （如果程序中图片路径不对，就修改）
Backtrace Log的获取方式:
cd waifu2x-ncnn-vulkan-python/src/build
gdb python
(gdb) b Waifu2x::~Waifu2x
(gdb) run waifu2x_ncnn_vulkan.py
运行直至crash
GDB crash backtrace:
Thread 1 "python" received signal SIGSEGV, Segmentation fault.
0x00007ffff282ab60 in ?? ()
(gdb) backtrace
#0 0x00007ffff282ab60 in ?? ()
#1 0x00007ffff69572ef in ncnn::VkBlobAllocator::fastFree (this=0x555555e2d400, ptr=0x555555e2ebe0)
at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/allocator.cpp:1045
#2 0x00007ffff6830b1d in ncnn::VkImageMat::release (this=0x555555c94d00) at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/mat.h:2217
#3 0x00007ffff6843830 in ncnn::VulkanDevicePrivate::destroy_dummy_buffer_image (this=0x555555c94bb0)
at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/gpu.cpp:1633
#4 0x00007ffff67bb4ca in ncnn::VulkanDevice::~VulkanDevice (this=<optimized out>, this=<optimized out>)
at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/gpu.cpp:2007
#5 0x00007ffff684341d in ncnn::destroy_gpu_instance () at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/gpu.cpp:1469
#6 0x00007ffff67b9b93 in ncnn::__ncnn_vulkan_instance_holder::~__ncnn_vulkan_instance_holder (this=<optimized out>, this=<optimized out>)
at /home/kodi/OpensourceProjects/waifu2x-ncnn-vulkan-python/src/ncnn/src/gpu.cpp:50
#7 0x00007ffff7a45db7 in __run_exit_handlers () from /usr/lib/libc.so.6
#8 0x00007ffff7a45f5e in exit () from /usr/lib/libc.so.6
#9 0x00007ffff7a2e159 in __libc_start_main () from /usr/lib/libc.so.6
#10 0x000055555555504e in _start ()