Let's analyze the code step by step to understand the CWE-476 (NULL Pointer Dereference) vulnerability:

1. Input analysis:
   - The function takes three parameters: buf, content, and glob.
   - We're particularly interested in the 'content' parameter, which is of type xmlElementContentPtr.

2. Initial check (line 3):
   - There's a null check for 'content': if (content == NULL) return;
   - This check prevents a null pointer dereference on 'content' itself.

3. Switch statement (line 6):
   - The code enters a switch statement based on content->type.
   - This access to content->type is safe due to the earlier null check.

4. Vulnerable line (line 18):
   - The code attempts to access content->c1->type.
   - While 'content' is checked for NULL, there's no check for content->c1 being NULL.

5. Potential vulnerability:
   - If content->c1 is NULL, attempting to access content->c1->type will result in a null pointer dereference.
   - This can lead to a program crash or undefined behavior.

Conclusion:
The vulnerability occurs because the code assumes that content->c1 is not NULL without explicitly checking. If content->c1 is NULL, accessing content->c1->type will cause a null pointer dereference, leading to the CWE-476 vulnerability. To fix this, an additional null check for content->c1 should be added before accessing its members.