vulkan : initialize vk_buffer_struct members to VK_NULL_HANDLE (ggml/893)

This prevents invalid frees when destroying a partially initialized
vk_buffer_struct. For example, this could happen in ggml_vk_create_buffer
when running out of device memory.

Co-authored-by: Tony Wasserka <neobrain@users.noreply.github.com>

ggml/src/ggml-vulkan.cpp

@@ -236,8 +236,8 @@ struct vk_device_struct {
 };
 
 struct vk_buffer_struct {
-    vk::Buffer buffer;
-    vk::DeviceMemory device_memory;
+    vk::Buffer buffer = VK_NULL_HANDLE;
+    vk::DeviceMemory device_memory = VK_NULL_HANDLE;
     vk::MemoryPropertyFlags memory_property_flags;
     void * ptr;
     size_t size = 0;