kunit: alloc_string_stream_fragment error handling bug fix

When it fails to allocate fragment, it does not free and return error.
And check the pointer inappropriately.

Fixed merge conflicts with
commit 61888776 ("kunit: update NULL vs IS_ERR() tests")
Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: YoungJun.park <her0gyugyu@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>