libsemanage/tests: return when str is NULL

CU_FAIL() does not stop the execution flow.

This issue has been found using Infer static analyzer.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsemanage/tests/test_utilities.c

@@ -142,6 +142,7 @@ void test_semanage_split_on_space(void)
 	if (!str) {
 		CU_FAIL
 		    ("semanage_split_on_space: unable to perform test, no memory");
+		return;
 	}
 	temp = semanage_split_on_space(str);
 	CU_ASSERT_STRING_EQUAL(temp, "bar    baz");