ffprobe: rework checking logic in writer_print_time() and writer_print_rational()

Make writer_print_time() and writer_print_rational() always call
writer_print_int() or writer_print_string(). This way the checks for
determining if the value should be printed or not are consistently
performed in the low level functions.

writer_print_rational() is moved downward in order to avoid a forward
reference.

Simplify/amend the overall logic.
