When `llama-batched-bench` is invoked _without_ setting `-npl`, "number
of parallel prompts", it segfaults.

The segfault is caused by invoking `max_element()` on a zero-length
vector, `n_pl`

This commit addresses that by first checking to see if the number of
parallel prompts is zero, and if so sets the maximum sequence size to 1;
otherwise, sets it to the original, the result of `max_element()`.

Fixes, when running `lldb build/bin/llama-batched-bench -- -m models/Meta-Llama-3-8B.gguf`

```
* thread ggerganov#1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x000000010000366c llama-batched-bench`main(argc=3, argv=0x000000016fdff268) at batched-bench.cpp:72:28
   69  	    llama_context_params ctx_params = llama_context_params_from_gpt_params(params);
   70
   71  	    // ensure enough sequences are available
-> 72  	    ctx_params.n_seq_max = *std::max_element(n_pl.begin(), n_pl.end());
```

Co-authored-by: compilade <git@compilade.net>