ggml-alloc: Don't try to re-use buffers of external tensors (#2562)

ggml-alloc: Don't try to re-use buffers of external tensors (#2562)

* ggml-alloc: Don't try to re-use buffers of external tensors

They might be weights that came from another context, so we
have no control over them (and they might be re-used elsewhere
so writing to them would be a bad idea).

* ggml-alloc: >= when checking for out-of-bounds

Co-authored-by: slaren <redacted>
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Co-authored-by: slaren <redacted>