BUILD: cli: clear a maybe-unused warning on some older compilers

BUILD: cli: clear a maybe-unused  warning on some older compilers

The SHOW_TOT() and SHOW_AVG() macros used in cli_io_handler_show_activity()
produce a warning on gcc 4.7 on MIPS with threads disabled because the
compiler doesn't know that global.nbthread is necessarily non-null, hence
that at least one iteration is performed. Let's just change the loop for
a do {} while () that lets the compiler know it's always initialized. It
also has the tiny benefit of making the code shorter.