kunit: make kunit_kfree(NULL) a no-op to match kfree()

The real kfree() function will silently return when given a NULL.
So a user might reasonably think they can write the following code:
  char *buffer = NULL;
  if (param->use_buffer) buffer = kunit_kzalloc(test, 10, GFP_KERNEL);
  ...
  kunit_kfree(test, buffer);

As-is, kunit_kfree() will mark the test as FAILED when buffer is NULL.
(And in earlier times, it would segfault).

Let's match the semantics of kfree().
Suggested-by: David Gow <davidgow@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/test.c

@@ -722,6 +722,9 @@ static inline bool kunit_kfree_match(struct kunit *test,
 
 void kunit_kfree(struct kunit *test, const void *ptr)
 {
+	if (!ptr)
+		return;
+
 	if (kunit_destroy_resource(test, kunit_kfree_match, (void *)ptr))
 		KUNIT_FAIL(test, "kunit_kfree: %px already freed or not allocated by kunit", ptr);
 }