To generate the patch for the given code slice with a CWE-787 vulnerability, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `size`:
  - `size` is obtained from `xmlStrlen(cur)`. We need to consider what values `size` might take.
  - If `xmlStrlen(cur)` returns a negative value, this could indicate an error or an invalid string length.

- 2: Allocation Size Calculation:
  - The allocation size is calculated as `(size + len + 1) * sizeof(xmlChar)`.
  - If `size` is negative, this could result in a very large allocation size, potentially leading to an integer overflow or an out-of-bounds allocation.

- 3: Potential Vulnerability:
  - The vulnerability arises if `size` is negative, which results in an incorrect allocation size calculation.
  - This is a classic example of an integer overflow vulnerability. Allocating memory with an incorrect size can lead to buffer overflows, crashes, or exploitable conditions.

### Conclusion:
The vulnerable behavior in this code is an integer overflow. It occurs because the code does not check if `size` is negative before using it to calculate the allocation size. If `size` is negative, the allocation size calculation will be incorrect, potentially leading to memory corruption and security vulnerabilities. Proper bounds checking should be added to ensure `size` is not negative.

### Step 2: Fixing Strategy

A common fixing strategy is to validate the `size` value before using it to calculate the allocation size. This can be done by adding a simple check to ensure `size` is not negative. If `size` is negative, the function can return an error or handle the situation accordingly.

Therefore, the patch should be:
```
<     size = xmlStrlen(cur);
<     ret = (xmlChar *)xmlRealloc(cur, (size + len + 1) * sizeof(xmlChar));
---
>     size = xmlStrlen(cur);
>     if (size < 0)
>     {
>         return (NULL);
>     }
>     ret = (xmlChar *)xmlRealloc(cur, (size + len + 1) * sizeof(xmlChar));
```
This patch adds a simple check to ensure `size` is not negative before using it to calculate the allocation size. If `size` is negative, the function returns `NULL` to indicate an error.