selftests/bpf: Fix possible NULL passed to memcpy() with zero size

Prevent sanitizer from complaining about passing NULL into memcpy(),
even if it happens with zero size.
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-9-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/core_reloc.c

@@ -881,7 +881,8 @@ void test_core_reloc(void)
 		data = mmap_data;
 
 		memset(mmap_data, 0, sizeof(*data));
-		memcpy(data->in, test_case->input, test_case->input_len);
+		if (test_case->input_len)
+			memcpy(data->in, test_case->input, test_case->input_len);
 		data->my_pid_tgid = my_pid_tgid;
 
 		link = bpf_program__attach_raw_tracepoint(prog, tp_name);