To generate the patch for the given code slice, we need to follow a structured approach to identify the root cause of the vulnerability and then apply an appropriate fix. Here are the detailed reasoning steps:

### Step 1: Root Cause Analysis

1. **Understanding the Code Context:**
   - The function `read_file` is designed to read a file associated with a smart card (`sc_card`).
   - It takes a path to the file (`str_path`), a pointer to a buffer (`data`), and a pointer to the length of the data (`data_len`).

2. **Identifying the Vulnerable Behavior:**
   - The code attempts to select a file using `sc_select_file` and assigns the result to `file`.
   - The size of the file is then used to determine the length (`len`) for reallocating the buffer `*data`.

3. **Potential Vulnerability:**
   - The line `len = file ? file->size : 4096;` sets `len` to `file->size` if `file` is not `NULL`, otherwise it defaults to `4096`.
   - If `file` is not `NULL` but `file->size` is `0`, the code will attempt to reallocate the buffer to a size of `0` bytes.
   - Calling `realloc` with a size of `0` can lead to undefined behavior. According to the C standard, `realloc` with a size of `0` may either return `NULL` or a unique pointer that can be safely passed to `free`. However, subsequent use of this pointer can lead to out-of-bounds access.

### Step 2: Fixing Strategy

1. **Ensuring Valid Allocation Size:**
   - To prevent the undefined behavior, we need to ensure that `len` is never set to `0`.
   - This can be achieved by adding a condition to check if `file->size` is greater than `0` before assigning it to `len`.

2. **Applying the Fix:**
   - Modify the assignment of `len` to ensure it is only set to `file->size` if `file` is not `NULL` and `file->size` is greater than `0`.
   - Otherwise, default to `4096`.

### Conclusion:
The vulnerability arises from the potential for `len` to be set to `0`, leading to undefined behavior when calling `realloc`. The fix ensures that `len` is only set to `file->size` if it is a positive value, otherwise, it defaults to `4096`.

### Patch:
```diff
< 	len = file ? file->size : 4096;
---
> 	len = file && file->size > 0 ? file->size : 4096;
```

### Reasoning Steps for the Patch:
1. **Check for Valid File and Size:**
   - The condition `file && file->size > 0` ensures that `file` is not `NULL` and `file->size` is a positive value.
2. **Assign Valid Length:**
   - If the condition is true, `len` is set to `file->size`.
   - If the condition is false, `len` defaults to `4096`.

By following these steps, we ensure that `len` is always a valid size for reallocation, thus preventing the CWE-787 (Out-of-bounds write) vulnerability.