context: zero output buffer on allocation (#20781)

context: zero output buffer on allocation (#20781)

* context: zero output buffer on allocation

Address GHSA-wqq9-25mr-rw76.

The logits output buffer allocated in output_reserve() uses
posix_memalign(), which does not zero memory. The buffer is only
written during decode when needs_raw_logits() returns true. When
backend samplers cover all output sequences, needs_raw_logits()
returns false and the buffer is never written, but
llama_get_logits() still returns a pointer to it, exposing stale
heap content.

Zero the buffer after allocation to prevent information disclosure
through the public logits API.

Found-by: Pwno
* Update src/llama-context.cpp

Co-authored-by: Georgi Gerganov <redacted>
---------

Co-authored-by: Georgi Gerganov <redacted>