kunit: fix too small allocation when using suite-only kunit.filter_glob

When a user filters by a suite and not a test, e.g.
$ ./tools/testing/kunit/kunit.py run 'suite_name'

it hits this code
  const int len = strlen(filter_glob);
  ...
  parsed->suite_glob = kmalloc(len, GFP_KERNEL);
which fails to allocate space for the terminating NULL.

Somehow, it seems like we can't easily reproduce this under UML, so the
existing `parse_filter_test()` didn't catch this.

Fix this by allocating `len + 1` and switch to kzalloc() just to be a
bit more defensive. We're only going to run this code once per kernel
boot, and it should never be very long.

Also update the unit tests to be a bit more cautious.
This bug showed up as a NULL pointer dereference here:
>  KUNIT_EXPECT_STREQ(test, (const char *)filtered.start[0][0]->name, "suite0");
`filtered.start[0][0]` was NULL, and `name` is at offset 0 in the struct,
so `...->name` was also NULL.

Fixes: 3b29021ddd10 ("kunit: tool: allow filtering test cases via glob")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/executor.c

@@ -33,7 +33,7 @@ static void kunit_parse_filter_glob(struct kunit_test_filter *parsed,
 	const char *period = strchr(filter_glob, '.');
 
 	if (!period) {
-		parsed->suite_glob = kmalloc(len, GFP_KERNEL);
+		parsed->suite_glob = kzalloc(len + 1, GFP_KERNEL);
 		parsed->test_glob = NULL;
 		strcpy(parsed->suite_glob, filter_glob);
 		return;

lib/kunit/executor_test.c

@@ -149,6 +149,7 @@ static void filter_suites_test(struct kunit *test)
 
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, filtered.start);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, filtered.start[0]);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, filtered.start[0][0]);
 	KUNIT_EXPECT_STREQ(test, (const char *)filtered.start[0][0]->name, "suite0");
 }