factor vs printf failure

[Jim Meyering]

Hi Pádraig,

I noticed that a recent change accumulates the return value of printf
into a variable of type size_t:

+static size_t n_out; /* How much data we've written to stdout.  */
+
...
-    printf ("%"PRIuMAX, t0);
+    n_out += printf ("%"PRIuMAX, t0);

The problem is that when printf fails, it returns a negative int,
which will be mapped to a very large size_t value in this case.
When printf fails we don't really care about how buffering
is done (that's the purpose of n_out), but it's worth at least
a comment, if only to forestall reports from static analyzers.

Thanks for all of your work,

Jim