Insufficient check for uninitialized storage pointer access

## Description

```solidity
contract Foo {
struct Hi {
uint256 hello;
}

function foo() internal returns (Hi storage ret) {
ret = ret;
ret.hello = 123;
}
}
```

This code accesses `ret` storage pointer without initializing it. It should not compile, but the Solidity compiler accepts this code.

## Environment

- Compiler version: `Version: 0.8.18+commit.87f61d96.Linux.g++`
- Target EVM version (as per compiler settings): Default setting
- Operating system: Ubuntu 22.04

## Steps to Reproduce

Save the above file as `test.sol` and run `solc test.sol`.

Without `ret = ret;` line, the expected warning message is printed:
```
Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: " to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
--> test.sol

Warning: Source file does not specify required compiler version! Consider adding "pragma solidity ^0.8.18;"
--> test.sol

Error: This variable is of storage pointer type and can be returned without prior assignment, which would lead to undefined behaviour.
--> test.sol:6:38:
|
6 | function foo() internal returns (Hi storage ret) {
| ^^^^^^^^^^^^^^

Error: This variable is of storage pointer type and can be accessed without prior assignment, which would lead to undefined behaviour.
--> test.sol:7:9:
|
7 | ret.hello = 123;
| ^^^
Note: The variable was declared here.
--> test.sol:6:38:
|
6 | function foo() internal returns (Hi storage ret) {
| ^^^^^^^^^^^^^^
```