Segfault when parsing grammars with left-recursion

[HanClinto]

Using latest master branch on OSX Sonoma 14.2.1 (23C71), M1 Macbook Pro.

When parsing grammars that contain left-recursion, the application parses with a segfault.

Example:

./main -m ./models/llama-2-7b.Q4_0.gguf --grammar "root ::= root \"a\" | \"b\""

Results in:

system_info: n_threads = 8 / 10 | AVX = 0 | AVX_VNNI = 0 | AVX2 = 0 | AVX512 = 0 | AVX512_VBMI = 0 | AVX512_VNNI = 0 | FMA = 0 | NEON = 1 | ARM_FMA = 1 | F16C = 0 | FP16_VA = 1 | WASM_SIMD = 0 | BLAS = 1 | SSE3 = 0 | SSSE3 = 0 | VSX = 0 | MATMUL_INT8 = 0 |
sampling:
	repeat_last_n = 64, repeat_penalty = 1.000, frequency_penalty = 0.000, presence_penalty = 0.000
	top_k = 40, tfs_z = 1.000, top_p = 0.950, min_p = 0.050, typical_p = 1.000, temp = 0.800
	mirostat = 0, mirostat_lr = 0.100, mirostat_ent = 5.000
sampling order:
CFG -> Penalties -> top_k -> tfs_z -> typical_p -> top_p -> min_p -> temperature
generate: n_ctx = 512, n_batch = 2048, n_predict = -1, n_keep = 1


[1]    50441 segmentation fault  ./main -m ./models/llama-2-7b.Q4_0.gguf --grammar

The segfault occurs in llama_grammar_init -- still digging into it, but wanted to at least log the bug.

This is not a regression, and is different from previous grammar-related segfault errors in the past.

This probably affects the server, and represents a vector to crash the server with a malformed request.

Ideally we should be able to catch left-recursion situations and hopefully either continue gracefully, or at the very least fail with an intelligent error message.