fix crash if messages key does not exist

Author: cjpais

llama.cpp/server/oai.h

@@ -35,7 +35,7 @@ inline static json oaicompat_completion_params_parse(
     // https://platform.openai.com/docs/api-reference/chat/create
     llama_sampling_params default_sparams;
     llama_params["model"]             = json_value(body, "model", std::string("unknown"));
-    llama_params["prompt"]            = format_chat(model, chat_template, body["messages"]);
+    llama_params["prompt"]            = format_chat(model, chat_template, body.at("messages"));
     llama_params["cache_prompt"]      = json_value(body, "cache_prompt", false);
     llama_params["temperature"]       = json_value(body, "temperature", 0.0);
     llama_params["top_k"]             = json_value(body, "top_k", default_sparams.top_k);