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
diff --git a/src/checks.c b/src/checks.c
index 7d42422..49bd886 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -3398,7 +3398,7 @@
 	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;
 	}