foreach: fixed maybe uninitialized warning in test (-O2)

foreach: fixed maybe uninitialized warning in test (-O2)

/home/rusty/devel/cvs/ccan/ccan/tap/tap.h:57:59: warning: ‘max_iters’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    _gen_result(1, __func__, __FILE__, __LINE__, "%s", #e) : \
                                                           ^
/home/rusty/devel/cvs/ccan/ccan/foreach/test/run-nested.c:47:17: note: ‘max_iters’ was declared here
  int i, j, sum, max_iters;

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>