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`:
  - The `size` parameter is passed directly to the `_stream->send` function without any validation.
  - If `size` is greater than the buffer size of the underlying stream, this could lead to an out-of-bounds write.

- 2: Potential Vulnerability:
  - The vulnerability arises if `size` exceeds the buffer size of the underlying stream.
  - This is a classic example of a buffer overflow vulnerability. Writing outside the bounds of the allocated buffer can corrupt adjacent memory, potentially leading to crashes or exploitable conditions.

- 3: Missing Validation:
  - The code does not perform any validation on the `size` parameter before passing it to the `_stream->send` function.
  - This lack of validation allows an attacker to potentially overflow the buffer by providing a large `size` value.

### Conclusion:
The vulnerable behavior in this code is a buffer overflow. It occurs because the `size` parameter is not validated before being passed to the `_stream->send` function. If `size` is greater than the buffer size of the underlying stream, the `_stream->send` function will write past the end of the buffer, leading to potential memory corruption and security vulnerabilities.

### Step 2: Fixing Strategy

- A common fixing strategy is to validate the `size` parameter before passing it to the `_stream->send` function.
- In this case, we can add a simple check to ensure that `size` is not zero before calling `_stream->send`. This is because a zero-sized write is a valid operation and does not pose a buffer overflow risk.
- By adding this check, we can prevent the buffer overflow vulnerability and ensure that the code behaves correctly even when `size` is zero.

The resulting patch is:
```
<     }
<     return _stream->send(_stream, data, size);
---
>     }
>     if (size == 0)
>     {
>         return 0;
>     }
>     return _stream->send(_stream, data, size);
```
This patch adds a simple validation check to ensure that `size` is not zero before calling `_stream->send`. If `size` is zero, the function returns immediately with a value of zero, preventing the buffer overflow vulnerability.