stm32/mbedtls: Add NULL pointer check in m_free_mbedtls.

According to the C standard the free(void *ptr) function: if ptr is a null
pointer, no action occurs.

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

ports/stm32/mbedtls/mbedtls_port.c

@@ -62,6 +62,9 @@ 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;