kunit: Protect string comparisons against NULL

Add NULL checks to KUNIT_BINARY_STR_ASSERTION() so that it will fail
cleanly if either pointer is NULL, instead of causing a NULL pointer
dereference in the strcmp().

A test failure could be that a string is unexpectedly NULL. This could
be trapped by KUNIT_ASSERT_NOT_NULL() but that would terminate the test
at that point. It's preferable that the KUNIT_EXPECT_STR*() macros can
handle NULL pointers as a failure.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>