aft: Silence format-overflow warning with gcc-10.

	aft.c: In function 'print_list_item':
	aft.c:736:17: warning: '%*u' directive writing between 1 and 65529 bytes into a region of size 30 [-Wformat-overflow=]
	  736 |   sprintf(buf, "%*u:%02u:%02d", max_width - 6, hours, mins,
	      |                 ^~~
	aft.c:736:16: note: directive argument in the range [1, 4294967295]
	  736 |   sprintf(buf, "%*u:%02u:%02d", max_width - 6, hours, mins,
	      |                ^~~~~~~~~~~~~~~
	aft.c:736:16: note: directive argument in the range [-59, 59]
	aft.c:736:3: note: 'sprintf' output between 8 and 65545 bytes into a destination of size 30
	  736 |   sprintf(buf, "%*u:%02u:%02d", max_width - 6, hours, mins,
	      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	  737 |    seconds % 60);
	      |    ~~~~~~~~~~~~~

The code is correct but gcc can't prove it. Silence the warning by
passing the size of the buffer and asserting that it won't overflow
before printing to it.