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 data buffer (`data`), and a pointer to the length of the data buffer (`data_len`).

2. **Analyzing the Vulnerable Line:**
   - The line in question is `p = realloc(*data, len);`.
   - `realloc` is used to resize the memory block pointed to by `*data` to `len` bytes.

3. **Determining the Value of `len`:**
   - `len` is assigned the value `file ? file->size : 4096;`.
   - This means `len` is set to `file->size` if `file` is not `NULL`, otherwise, it is set to 4096.

4. **Potential Vulnerability:**
   - If `file` is not `NULL` but `file->size` is 0, `len` will be set to 0.
   - Calling `realloc` with a size of 0 can lead to undefined behavior. According to the C standard, if the size is 0, `realloc` may return `NULL` and the original block may be freed, leading to potential use-after-free or double-free vulnerabilities.

### Step 2: Fixing Strategy

1. **Ensuring Valid `len` Value:**
   - We need to ensure that `len` is never set to 0 when calling `realloc`.
   - 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, set `len` to 4096.

### Conclusion:
The vulnerability arises from the potential assignment of 0 to `len`, which can lead to undefined behavior when calling `realloc`. The fix ensures that `len` is only set to `file->size` if it is greater than 0, 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 `file` and `file->size`:**
   - The condition `file && file->size > 0` ensures that `file` is not `NULL` and `file->size` is greater than 0.
   
2. **Assign `len` Appropriately:**
   - If both conditions are true, `len` is assigned `file->size`.
   - If either condition is false, `len` is assigned 4096.

By following these steps, we ensure that `len` is never 0 when calling `realloc`, thus preventing the CWE-787 (Out-of-bound write) vulnerability.