vulkan : improve ggml_vk_create_buffer error handling

[FanShupei]

This PR solves a segfault on Mali GPU. Sorry I can't give an easy reproduction since I can't reproduce it on my desktop GPU.

In ggml_vk_create_buffer, when device->device.createBuffer throws an exception (e.g. out of memory if size is too large), the program will crash. Note in this case buf->size is already set but buf->device is not set, the destructor of vk_buffer_struct will cause trouble since its device is invalid.

The PR solves the problem by setting buf->size only at the end of success path. Now when the context length is too long or model is too large, it will print a proper error message instead of segfault.

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