The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in the given code slice is:

The code attempts to access the 'type' member of 'content->c1' without first checking if 'content->c1' is NULL. This can lead to a NULL pointer dereference if 'content->c1' is indeed NULL.

The vulnerability occurs because:

1. The function checks if 'content' is NULL at the beginning (line 3), but it doesn't check if 'content->c1' is NULL before accessing it.

2. The code assumes that if 'content' is not NULL, then 'content->c1' will also be valid, which may not always be the case.

3. There's no defensive programming to ensure that 'content->c1' is not NULL before attempting to access its 'type' member.

To fix this vulnerability, a NULL check should be added for 'content->c1' before accessing its members. This would prevent the potential NULL pointer dereference and make the code more robust.