chat-peg-parser : clear current_tool when pending_tool_call is reset (#29942)

A TOOL_ID node that arrives after TOOL_CLOSE wrote through `current_tool`,
which still pointed into the just-destroyed `pending_tool_call` optional
(use-after-free, then a second free of the id buffer). Clear the pointer on
reset.

common/chat-peg-parser.cpp

@@ -451,6 +451,7 @@ void common_chat_peg_mapper::map(const common_peg_ast_node & node) {
                 result.tool_calls.push_back(pending_tool_call.value());
             }
             pending_tool_call.reset();
+            current_tool = nullptr;
         }
     }
 }