To generate the patch for the CWE-190 vulnerability in the given code slice, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `size`:
  - The `size` parameter is of type `int`, which is a signed integer type.
  - This means that `size` can potentially hold negative values.

- 2: Potential Vulnerability:
  - The vulnerability arises if `size` is a negative number.
  - Passing a negative value to a memory allocation function like `bson_realloc_func` can lead to undefined behavior, potentially causing the program to crash or behave unexpectedly.

- 3: CWE-190:
  - CWE-190 is a vulnerability that occurs when a function is called with a negative integer argument that is used as a size or index.
  - In this case, the `size` parameter is used as the size argument to `bson_realloc_func`, making it a potential target for CWE-190.

### Conclusion:
The vulnerable behavior in this code is a CWE-190 vulnerability. It occurs because the `size` parameter is of type `int`, which allows it to hold negative values. Passing a negative value to `bson_realloc_func` can lead to undefined behavior. To fix this vulnerability, we need to ensure that the `size` parameter cannot hold negative values.

### Step 2: Fixing Strategy

- A common fixing strategy for CWE-190 is to change the type of the `size` parameter to an unsigned integer type, such as `size_t`.
- `size_t` is an unsigned integer type that is guaranteed to be large enough to hold the size of any object.
- By changing the type of `size` to `size_t`, we can ensure that it cannot hold negative values, preventing the CWE-190 vulnerability.

Therefore, the patch should be:
```
< void *bson_realloc( void *ptr, int size ) {
---
> void *bson_realloc( void *ptr, size_t size ) {
```
This change ensures that the `size` parameter cannot hold negative values, preventing the CWE-190 vulnerability.