Add mutex and NULL checks to secure UDP pacing queue access

• Added a dedicated mutex (udp_pacing_queue_mutex) to protect concurrent access to the UDP pacing queue in rist_common_ctx.
• Secured udp_pacing_pthread by locking the mutex during queue operations.
• Added explicit NULL checks for udp_pacing_buffer, its data, and peer fields to prevent crashes due to bad pointer dereference.
• Improved error logging for invalid or empty string copies in URL and peer config parsing.
• Ensured all string copies are properly bounded and null-terminated.
• Added cleanup log for NULL buffers during UDP pacing queue cleanup. These changes improve the robustness and thread safety of the UDP pacing mechanism.