Failure when selecting non-zero GPU index with multiple GPUs

[Masa-tam]

When using whisper.cpp version 1.7.5 in an environment with multiple GPUs, specifying a gpu_device other than 0 causes an assertion failure and termination.
This occurs in make_buft_list, which creates a list of device-buffer pairs.
There's an issue in the code that attempts to use GPU 0 when the specified gpu_device is greater than 0 and that specific GPU is not present in the environment.
For example, in an environment with two GPUs, if gpu_device == 1 is set, make_buft_list incorrectly registers device-buffer pairs for both GPU 0 and GPU 1.
While the device selection correctly chooses GPU 1, the buffer selection process only checks the device type and the buffer's device type, without considering the actual device ID.
Consequently, it selects the buffer associated with GPU 0, which is the first entry in the buffer list.
This mismatch between the execution device (GPU 1) and the buffer's device (GPU 0) leads to the assertion failure.
Although it's a very crude hack, inserting the following code starting at line 1406 in whisper.cpp, right after the GPU loop finishes, allows the program to work correctly:

        if (buft_list.size() > 1)
        {
            buft_list.erase(buft_list.begin());
        }

This code forcibly removes the entry for GPU 0 from buft_list if both the GPU 0 entry and the specified GPU entry exist.

[peardox]

A lot of the code that checks devices blindly ignores null pointers

e.g. whisper_backend_init_gpu

    if (params.use_gpu) {
        for (size_t i = 0; i < ggml_backend_dev_count(); ++i) {
            ggml_backend_dev_t dev_cur = ggml_backend_dev_get(i);
            if(dev_cur == nullptr) {
                continue;
            }
            if (ggml_backend_dev_type(dev_cur) == GGML_BACKEND_DEVICE_TYPE_GPU) {

I added the nullptr continue in a PR so you won't have it but it is possible to get a null back from the dev_get call for example

There's stuff like this all over the place, especially in ggml
e.g. pass this a nullptr = Core Cump

ggml_backend_dev_t ggml_backend_buft_get_device(ggml_backend_buffer_type_t buft) {
    return buft->device;
}

I was told off for fixing some of those..:(