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>

include/kunit/test.h

@@ -758,7 +758,7 @@ do {									       \
 		.right_text = #right,					       \
 	};								       \
 									       \
-	if (likely(strcmp(__left, __right) op 0))			       \
+	if (likely((__left) && (__right) && (strcmp(__left, __right) op 0)))   \
 		break;							       \
 									       \
 									       \