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 <[e-mail]>
Fixes: 6d696c4695c5 ("kunit: add ability to run tests after boot using debugfs")
Reviewed-by: Rae Moar <[e-mail]>
Acked-by: David Gow <[e-mail]>
Reviewed-by: Muhammad Usama Anjum <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/kunit/test.c

···
 	kunit_debugfs_create_suite(suite);
 	suite->status_comment[0] = '\0';
 	suite->suite_init_err = 0;
-	string_stream_clear(suite->log);
+
+	if (suite->log)
+		string_stream_clear(suite->log);
 }
 
 bool kunit_enabled(void)