kunit: update NULL vs IS_ERR() tests

The alloc_string_stream() functions were changed from returning NULL on
error to returning error pointers so these caller needs to be updated
as well.

Fixes: 78b1c6584fce ("kunit: string-stream: Simplify resource use")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Daniel Latypov <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/kunit/string-stream.c

···
 	frag_container = alloc_string_stream_fragment(stream->test,
 						      len,
 						      stream->gfp);
-	if (!frag_container)
-		return -ENOMEM;
+	if (IS_ERR(frag_container))
+		return PTR_ERR(frag_container);
 
 	len = vsnprintf(frag_container->fragment, len, fmt, args);
 	spin_lock(&stream->lock);

lib/kunit/test.c

···
 	kunit_set_failure(test);
 
 	stream = alloc_string_stream(test, GFP_KERNEL);
-	if (!stream) {
+	if (IS_ERR(stream)) {
 		WARN(true,
 		     "Could not allocate stream to print failed assertion in %s:%d\n",
 		     loc->file,