To generate the patch for the CWE-476 (NULL Pointer Dereference) vulnerability, we need to follow a structured approach to identify the root cause and then apply an appropriate fix. Here are the reasoning steps:

### Step 1: Root Cause Analysis

1. **Identify the Vulnerable Code:**
   - The code in question is:
     ```c
     1 int rval_get_tmp_str(struct run_act_ctx *h, struct sip_msg *msg, str *tmpv, struct rvalue *rv, struct rval_cache *cache, struct rval_cache *tmp_cache)
     5     switch (rv->type)
     ```
   - The vulnerability is identified at line 5: `switch (rv->type)`.

2. **Understand the Context:**
   - The function `rval_get_tmp_str` takes several parameters, one of which is a pointer to an `rvalue` structure (`struct rvalue *rv`).
   - The `switch` statement is attempting to access the `type` member of the `rv` structure.

3. **Potential Issue:**
   - If the `rv` pointer is `NULL`, dereferencing `rv->type` will result in a NULL pointer dereference, leading to undefined behavior, crashes, or potential security vulnerabilities.

4. **Determine the Vulnerability:**
   - The vulnerability occurs because there is no check to ensure that `rv` is not `NULL` before accessing its members.
   - This is a classic example of CWE-476: NULL Pointer Dereference.

### Step 2: Fixing Strategy

1. **Add a NULL Check:**
   - To prevent the NULL pointer dereference, we need to add a check to ensure that `rv` is not `NULL` before accessing its members.
   - If `rv` is `NULL`, the function should handle this case appropriately, such as by returning an error code.

2. **Implement the Fix:**
   - The fix involves adding an `if` statement to check if `rv` is `NULL` and returning an error code if it is.
   - The modified code should look like this:
     ```c
     <     int i;
     <     switch (rv->type)
     ---
     >     int i;
     >     if (rv == NULL)
     >     {
     >         return -1;
     >     }
     >     switch (rv->type)
     ```

### Conclusion

The patch ensures that the function `rval_get_tmp_str` checks if the `rv` pointer is `NULL` before attempting to access its `type` member. If `rv` is `NULL`, the function returns an error code (`-1`), preventing the NULL pointer dereference and mitigating the vulnerability.

By following these reasoning steps, we can systematically identify the root cause of the vulnerability and apply an appropriate fix to ensure the code is robust and secure.