LIB: Fix potential NULL pointer dereference

In tfm_vprintf.c, if the calculate_length flag is set and the string
passed to output_str is NULL, then the program would dereference the
NULL pointer as per the following warning:

warning: potential null pointer dereference [-Wnull-dereference]
   64 |         while (*str_ptr++ != '\0') {
      |                ^~~~~~~~~~

Fix this by adding an assertion that the string is non-NULL and
returning from the function in the case of release builds.

Change-Id: Ibc9fe50a19c8255298ea69b22770f8a255ad9266
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>
(cherry picked from commit 3e369bd)
Signed-off-by: Stefan Gloor <stefan.gloor@siemens.com>

Author: jackson-cooperd

lib/tfm_vprintf/src/tfm_vprintf.c

@@ -59,6 +59,11 @@ static void output_str(tfm_log_output_str output_func, void *priv, const char *s
 {
     const char *str_ptr = str;
 
+    if (str_ptr == NULL) {
+        assert(false);
+        return;
+    }
+
     if (calculate_length) {
         len = 0;
         while (*str_ptr++ != '\0') {