fix potential NULL array printing

CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to this project will be documented in this file.
 This project uses the changelog in accordance with [keepchangelog](http://keepachangelog.com/). Please use this to write notable changes, which is not the same as git commit log...
 
 ## [unreleased][unreleased]
+ - Fixed potential NULL array printing (@jmichel)
  - Added PIV aid to resource file (@jmichel)
  - Fixed failing compilation on Proxspace environment due to how python is initialized (@jmichel)
  - Fixed length check in sim module communications (@jmichel)

client/src/util.c

@@ -303,7 +303,8 @@ void print_hex_noascii_break(const uint8_t *data, const size_t len, uint8_t brea
 
 static void print_buffer_ex(const uint8_t *data, const size_t len, int level, uint8_t breaks) {
 
-    if (len < 1)
+    // sanity checks
+    if ((data == NULL) || (len < 1))
         return;
 
     char buf[UTIL_BUFFER_SIZE_SPRINT + 3];