grammar: add test case for nullable symbol loop

Reproduce stack overflow (or OOM) with ( [x]* )* found while adding
GBNF support to ripgrep-edit.

llama-server reproducer:

curl \
  -X POST \
  -d '{
    "messages": [{ "role": "user", "content": "write yes" }],
    "grammar": "root ::= ( [x]* )*"
  }' \
  -H "Content-Type: application/json" \
  http://localhost:8811/v1/chat/completions

tests/test-grammar-integration.cpp

@@ -784,6 +784,24 @@ static void test_quantifiers() {
             "0xFF 0x12 0xAB 0x00 0x00 0x00",
         }
     );
+    test_grammar(
+        "segfault",
+        // Grammar
+        R"""(
+            root ::= ( [x]* )*
+        )""",
+        // Passing strings
+        {
+            "",
+            "x",
+            "xx"
+        },
+        // Failing strings
+        {
+            "y",
+            "yy"
+        }
+    );
 }
 
 static void test_failure_missing_root() {