Fix GCC build warning

Output from build:

mainwindow.cpp: In member function ‘void MainWindow::OnPositionChanged(qint64)’:
mainwindow.cpp:219:53: warning: ‘__builtin___snprintf_chk’ output may be truncated before the last format character [-Wformat-truncation=]
  219 |             "%lld:%02lld:%02lld / %lld:%02lld:%02lld",
      |                                                     ^
In file included from /usr/include/stdio.h:867,
                 from /usr/include/c++/9/cstdio:42,
                 from /usr/include/c++/9/ext/string_conversions.h:43,
                 from /usr/include/c++/9/bits/basic_string.h:6493,
                 from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from mainwindow.h:4,
                 from mainwindow.cpp:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:67:35: note: ‘__builtin___snprintf_chk’ output between 18 and 59 bytes into a destination of size 20
   67 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   68 |        __bos (__s), __fmt, __va_arg_pack ());
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~