commit 25b453fd4d354cd16de7ff233da626758c6ab1be
Author: Kaspar Schleiser <kaspar@schleiser.de>
Date:   Wed Jan 10 14:53:55 2018 +0100

    fixup! sys/fmt: add new byte to hex converters:

diff --git a/sys/fmt/fmt.c b/sys/fmt/fmt.c
index 3d333195e..36540a545 100644
--- a/sys/fmt/fmt.c
+++ b/sys/fmt/fmt.c
@@ -64,11 +64,11 @@ size_t fmt_byte_hex(char *out, uint8_t byte)

 size_t fmt_bytes_hex(char *out, const uint8_t *ptr, size_t n)
 {
-    size_t len = 0;
+    size_t len = n * 2;
+
     if (out) {
         while (n--) {
             out += fmt_byte_hex(out, *ptr++);
-            len += 2;
         }
     }