BUG/MINOR: sendmail: The return of vsnprintf is not cleanly tested

The string formatted by vsnprintf may be bigger than the size of
the buffer "buf". This case is not tested.

This sould be backported to 1.6 and 1.7

src/checks.c

@@ -3398,7 +3398,7 @@ void send_email_alert(struct server *s, int level, const char *format, ...)
 	len = vsnprintf(buf, sizeof(buf), format, argp);
 	va_end(argp);
 
-	if (len < 0) {
+	if (len < 0 || len >= sizeof(buf)) {
 		Alert("Email alert [%s] could not format message\n", p->id);
 		return;
 	}