### Step 1: Root Cause Analysis

#### Understanding the Code:
1. **Function Purpose**: The function `initiate_stratum` is responsible for initiating a connection to a pool server, sending a subscription request, and processing the response.
2. **Memory Allocation**: The function allocates memory dynamically using `strdup` to store strings received from the server.
3. **Potential Memory Leak**: The line `pool->nonce1 = strdup(json_string_value(json_array_get(res_val, 1)));` allocates memory for `pool->nonce1` without checking if `pool->nonce1` already points to previously allocated memory.

#### Identifying the Vulnerability:
- **CWE-401: Memory Leak**: If `pool->nonce1` already contains a pointer to previously allocated memory, calling `strdup` again without freeing the old memory will result in a memory leak. This is because the old memory will no longer be accessible, and thus cannot be freed.

### Step 2: Fixing Strategy

#### Strategy:
- **Free Previously Allocated Memory**: Before assigning a new value to `pool->nonce1`, ensure that any previously allocated memory is freed. This prevents memory leaks by ensuring that the old memory is properly deallocated before the new memory is assigned.

#### Implementation:
- **Add a `free` Call**: Insert a call to `free(pool->nonce1)` before reassigning `pool->nonce1` with the result of `strdup`.

### Conclusion:
The patch ensures that any previously allocated memory for `pool->nonce1` is freed before assigning new memory to it. This prevents memory leaks by ensuring that the old memory is properly deallocated.

### Patch:
```diff
<     }
<     pool->nonce1 = strdup(json_string_value(json_array_get(res_val, 1)));
---
>     }
>     free(pool->nonce1);
>     pool->nonce1 = strdup(json_string_value(json_array_get(res_val, 1)));
```

### Detailed Reasoning Steps:
1. **Identify the Vulnerable Line**: The line `pool->nonce1 = strdup(json_string_value(json_array_get(res_val, 1)));` is identified as the source of the memory leak.
2. **Understand the Allocation**: `strdup` allocates memory for a copy of the string returned by `json_string_value`.
3. **Check for Existing Allocation**: Before assigning the new value, check if `pool->nonce1` already points to allocated memory.
4. **Free Existing Memory**: If `pool->nonce1` is not `NULL`, free the existing memory to prevent a memory leak.
5. **Assign New Value**: After freeing the old memory, assign the new value to `pool->nonce1`.

By following these steps, the patch ensures that memory is managed correctly, preventing memory leaks and adhering to best practices for dynamic memory management.