kunit: Fix NULL-dereference in kunit_init_suite() if suite->log is NULL

suite->log must be checked for NULL before passing it to
string_stream_clear(). This was done in kunit_init_test() but was missing
from kunit_init_suite().

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: 6d696c4695c5 ("kunit: add ability to run tests after boot using debugfs")
Reviewed-by: Rae Moar <rmoar@google.com>
Acked-by: David Gow <davidgow@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>