gguf : reject empty metadata key names during parsing

[rxho]

Summary

Fix #20873 — malformed GGUF files with zero-length key strings trigger GGML_ASSERT(!key.empty()) in the gguf_kv constructor, causing an abort. The parser should reject such files gracefully with an error log instead.

Root Cause

In ggml/src/gguf.cpp, gguf_init_from_file_ptr reads a key string from the file and passes it directly to the gguf_kv constructor without checking for empty keys. The constructor asserts non-empty, which aborts in debug builds and is UB in release.

There is already validation for duplicate keys and bad key sizes, but no check for the zero-length key case.

Fix

Add a guard immediately after the key is read (after the existing bad_alloc catch), before the duplicate-key loop:

if (ok && key.empty()) {
    GGML_LOG_ERROR("%s: empty key name at KV pair %" PRIi64 "\n", __func__, i);
    ok = false;
}

This follows the same pattern as the existing duplicate-key validation.

Changes

File	Change
ggml/src/gguf.cpp	Reject empty key names with error log
tests/test-gguf.cpp	3 new handcrafted test cases

Test Plan

Three new handcrafted GGUF file types exercise the fix:

Test case	Description
KV_EMPTY_KEY	All KV pairs have empty keys
KV_EMPTY_KEY_ONLY_FIRST	Only the first KV pair has an empty key
KV_EMPTY_KEY_ONLY_MID	A middle KV pair has an empty key

All three should fail gracefully (parser returns nullptr / error count incremented) without triggering GGML_ASSERT.

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