fix: null pointer dereference when loading malformed LoHa file

[nmouha]

Summary

In get_loha_weight_diff(), a null pointer dereference occurs when hada_t1 is present but hada_w1_a is missing, or when hada_t2 is present but hada_w2_a is missing.

Related Issue / Discussion

The nullptr check at line 368 (if (hada_1_up == nullptr || ...) is never reached because the crash happens first.

Additional Information

To reproduce the crash:

printf '\x8a\x00\x00\x00\x00\x00\x00\x00{"model.diffusion_model.input_blocks.1.1.transformer_blocks.0.attn1.to_q.hada_t1": {"dtype": "F32", "shape": [1], "data_offsets": [0, 4]}}AAAA' > /tmp/bad.safetensors
./bin/sd-cli -m ../models/v1-5-pruned-emaonly.safetensors --lora-model-dir /tmp -p "<lora:bad.safetensors:1.0>" -o /tmp/output.png

Checklist

• I have read and confirmed this PR follows the contribution guidelines.