Fix buffer overflow in graph timecode generation

When option `-c` is enabled (print timecode on x-axis in graph) and Shotdetect is compiled with any compiler optimization (`-O1`) and higher, the program always crashes with segmentation fault or buffer overflow.

The source of the problem is the string `char timecode[10]` – the resulting string from `sprintf` is exactly 10 characters long, leaving no space for null terminator.