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 `ptr_len`:
  - `ptr_len` is calculated as `src_len - 1 - byte * 2`. We need to consider what values `ptr_len` might take.
  - If `ptr_len` is negative, this could lead to an out-of-bounds access in the `init_get_bits` function.

- 2: Function Call and Buffer Size:
  - The `init_get_bits` function is called with `ptr` and `ptr_len * 8` as arguments.
  - The size of the buffer `ptr` points to is determined by `ptr_len`. If `ptr_len` is negative, this could result in an out-of-bounds access.

- 3: Potential Vulnerability:
  - The vulnerability arises if `ptr_len` is negative, which results in an invalid buffer size being passed to `init_get_bits`.
  - This is a classic example of a buffer overflow vulnerability. Passing an invalid buffer size can lead to undefined behavior, potentially causing crashes or exploitable conditions.

### Conclusion:
The vulnerable behavior in this code is a buffer overflow. It occurs because the code does not ensure that `ptr_len` is non-negative before passing it to `init_get_bits`. If `ptr_len` is negative, the function call will result in an invalid buffer size, leading to potential memory corruption and security vulnerabilities. Proper bounds checking should be added to ensure `ptr_len` is non-negative.

### Step 2: Fixing Strategy

A common fixing strategy is to add a check to ensure that `ptr_len` is non-negative before calling `init_get_bits`. This can be done by adding a conditional statement to check if `ptr_len` is less than 0. If it is, an error should be returned to prevent the potential buffer overflow.

Therefore, the patch should be:
```
<     GetBitContext gb;
<     init_get_bits(&gb, ptr, ptr_len * 8);
---
>     GetBitContext gb;
>     if (ptr_len < 0)
>     {
>         return AVERROR_INVALIDDATA;
>     }
>     init_get_bits(&gb, ptr, ptr_len * 8);
```
This patch adds a check to ensure that `ptr_len` is non-negative before calling `init_get_bits`, preventing the potential buffer overflow vulnerability.