transport-helper: replace checked snprintf with xsnprintf

We can use xsnprintf to do our truncation check with less
code. The error message isn't as specific, but the point is
that this isn't supposed to trigger in the first place
(because our buffer is big enough to handle any int).
Signed-off-by: NJeff King <peff@peff.net>

transport-helper.c

@@ -347,14 +347,11 @@ static int set_helper_option(struct transport *transport,
 static void standard_options(struct transport *t)
 {
 	char buf[16];
-	int n;
 	int v = t->verbose;
 
 	set_helper_option(t, "progress", t->progress ? "true" : "false");
 
-	n = snprintf(buf, sizeof(buf), "%d", v + 1);
-	if (n >= sizeof(buf))
-		die("impossibly large verbosity value");
+	xsnprintf(buf, sizeof(buf), "%d", v + 1);
 	set_helper_option(t, "verbosity", buf);
 
 	switch (t->family) {