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>

examples/gbnf-validator/gbnf-validator.cpp

@@ -101,7 +101,9 @@ int main(int argc, char** argv) {
     auto grammar = llama_grammar_init(
             grammar_rules.data(),
             grammar_rules.size(), parsed_grammar.symbol_ids.at("root"));
-
+    if (grammar == nullptr) {
+        throw std::runtime_error("Failed to initialize llama_grammar");
+    }
     // Read the input file
     std::string input_str;
     {