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>
diff --git a/src/cli.c b/src/cli.c
index da18149..a65db80 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1368,8 +1368,10 @@
 		unsigned int _v[MAX_THREADS];				\
 		unsigned int _tot;					\
 		const unsigned int _nbt = global.nbthread;		\
-		for (_tot = t = 0; t < _nbt; t++)			\
+		_tot = t = 0;						\
+		do {							\
 			_tot += _v[t] = (x);				\
+		} while (++t < _nbt);					\
 		if (_nbt == 1) {					\
 			chunk_appendf(&trash, " %u\n", _tot);		\
 			break;						\
@@ -1386,8 +1388,10 @@
 		unsigned int _v[MAX_THREADS];				\
 		unsigned int _tot;					\
 		const unsigned int _nbt = global.nbthread;		\
-		for (_tot = t = 0; t < _nbt; t++)			\
+		_tot = t = 0;						\
+		do {							\
 			_tot += _v[t] = (x);				\
+		} while (++t < _nbt);					\
 		if (_nbt == 1) {					\
 			chunk_appendf(&trash, " %u\n", _tot);		\
 			break;						\