whisper_init_from_buffer_with_params_no_state installs a read callback that
copies from buf->buffer + current_offset. When the buffer is exhausted (or the
supplied buffer is empty), size_to_copy is 0 and the source pointer can be null;
passing a null pointer to memcpy is undefined behavior even for a zero-length
copy (UBSan: 'null pointer passed as argument 2' at the memcpy). Loading a
crafted/short model through the buffer loader could hit this.
Skip the memcpy when there is nothing to copy. Loading from a null/empty or
truncated buffer now fails gracefully (returns NULL) with no UB.
This addresses bug 1 of #3879. Bug 2 (integer overflow when sizing the mel
filter buffer) is covered by the open PR #3780.
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