I'll provide the reasoning steps for generating this patch following the two-step process:

Step 1. Root Cause Analysis:

1. Input Analysis:
   - The function takes a pointer to data and a size parameter.
   - There's no validation of the 'size' parameter.

2. Function Behavior:
   - The function checks if the stream is closed but doesn't handle the case where 'size' might be zero.
   - It directly calls _stream->send() with the given parameters.

3. Potential Vulnerability:
   - If 'size' is zero, it might lead to undefined behavior in the _stream->send() function.
   - Some implementations might interpret a zero-size write as a request to write to a null pointer, potentially causing a buffer overflow.

4. CWE-787 Context:
   - CWE-787 is about out-of-bounds writes.
   - In this case, a zero-size write could be interpreted as writing to an invalid location, depending on the implementation of _stream->send().

Conclusion:
The vulnerability arises from not handling the case where 'size' is zero before calling _stream->send(). This could lead to undefined behavior or potential out-of-bounds writes, depending on how _stream->send() handles zero-size inputs.

Step 2. Fixing Strategy:

1. Add a check for zero-size input:
   - Before calling _stream->send(), we should check if 'size' is zero.
   - If 'size' is zero, we can safely return 0 without calling _stream->send().

2. Implement the fix:
   - Add an if statement to check if size is zero.
   - If size is zero, return 0 immediately.
   - Only call _stream->send() if size is non-zero.

3. Resulting patch:
```
<     }
<     return _stream->send(_stream, data, size);
---
>     }
>     if (size == 0)
>     {
>         return 0;
>     }
>     return _stream->send(_stream, data, size);
```

This patch ensures that zero-size writes are handled safely, preventing potential undefined behavior or out-of-bounds writes in the _stream->send() function.