ggml_allocr_new null pointer exception

[Light Lin]

GGML commit version: fc9e9558d9e90e8f943bae1cb622fd6113431a9a
Below is the minimal reproducible code

ggml_init_params params = {
   .mem_size   = 1024*1024,
   .mem_buffer = NULL,
};
auto ctx = ggml_init(params);
size_t allocr_buffer_size = 1024;
std::vector<char> allocr_buffer;
allocr_buffer.resize(allocr_buffer_size);
auto allor = ggml_allocr_new(allocr_buffer.data(), allocr_buffer_size, 32);
ggml_free(ctx);

// File: ggml-backend.c
size_t ggml_backend_buffer_get_alignment(ggml_backend_buffer_t buffer) {
    // crash at here, buffer->backend is NULL
    return ggml_backend_get_alignment(buffer->backend);
}