mimxrt/mbedtls: Add NULL pointer check in m_free_mbedtls.

Signed-off-by: Peter Züger <zueger.peter@icloud.com>

Author: peterzuger

ports/mimxrt/mbedtls/mbedtls_port.c

@@ -64,6 +64,9 @@ void *m_calloc_mbedtls(size_t nmemb, size_t size) {
 }
 
 void m_free_mbedtls(void *ptr_in) {
+ if (ptr_in == NULL) {
+ return;
+ }
  void **ptr = &((void **)ptr_in)[-2];
  #if DEBUG
  uint32_t nb;