model: assert nextn_predict_layers to prevent underflow (#20783)
Address GHSA-645x-v54x-34w8.
When nextn_predict_layers >= n_layer, n_layer - nextn_predict_layers
can underflow (unsigned wrap), which corrupts n_layer_kv_from_start.
Assert nextn_predict_layers immediately after parsing the GGUF key.
Found-by: Pwno