llama : return nullptr from llama_grammar_init (#8093)

* llama : return nullptr from llama_grammar_init

This commit updates llama_grammar_init to return nullptr instead of
throwing an exception.

The motivation for this is that this function is declared inside an
extern "C" block and is intended/may be used from C code which will not
be able to handle exceptions thrown, and results in undefined behavior.

On Windows and using MSVC the following warning is currently generated:
```console
C:\llama.cpp\llama.cpp(13998,1): warning C4297: 'llama_grammar_init':
function assumed not to throw an exception but does
C:\llama.cpp\llama.cpp(13998,1): message :
__declspec(nothrow), throw(), noexcept(true), or noexcept was specified
on the function
```

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* squash! llama : return nullptr from llama_grammar_init

Add checks for nullptr when calling llama_grammar_init.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

---------

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>
Co-authored-by: Clint Herron <hanclinto@gmail.com>

llama.cpp

@@ -14500,7 +14500,8 @@ struct llama_grammar * llama_grammar_init(
             continue;
         }
         if (llama_grammar_detect_left_recursion(vec_rules, i, &rules_visited, &rules_in_progress, &rules_may_be_empty)) {
-            throw std::runtime_error(format("unsupported grammar, left recursion detected for nonterminal at index %zu", i));
+            LLAMA_LOG_ERROR("unsupported grammar, left recursion detected for nonterminal at index %zu", i);
+            return nullptr;
         }
     }