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.

(cherry picked from commit 97b5d07a3e5a33552327bac2e4c9c6a0496f7b5e)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit b9aac36d8d8d3bb05c785aa8f6630338d078f64a)
Signed-off-by: Willy Tarreau <w@1wt.eu>