whisper : guard null source in buffer loader read callback (#3982)

* whisper : guard null source in buffer loader read callback

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.

* fixup! whisper : guard null source in buffer loader read callback

---------

Co-authored-by: Ben Younes <2910651+ousamabenyounes@users.noreply.github.com>

src/parakeet.cpp

@@ -3109,7 +3109,9 @@ struct parakeet_context * parakeet_init_from_buffer_with_params_no_state(void *
 
         size_t size_to_copy = buf->current_offset + read_size < buf->size ? read_size : buf->size - buf->current_offset;
 
-        memcpy(output, buf->buffer + buf->current_offset, size_to_copy);
+        if (size_to_copy > 0 && buf->buffer != nullptr) {
+            memcpy(output, buf->buffer + buf->current_offset, size_to_copy);
+        }
         buf->current_offset += size_to_copy;
 
         return size_to_copy;