Fix DirectInput data queries while GUI has focus (multitheftauto#5038)

## **Summary**

Added a null check before the DirectInput proxy clears buffered event
data.

Null-output event count and flush queries now return normally while the
GUI owns input. Calls that provide an event array are still cleared so
GTA does not receive input intended for the GUI.

## **Motivation**

`IDirectInputDevice8::GetDeviceData` allows the output buffer to be null
when checking how many events are waiting or flushing them. The proxy
forwarded these valid calls, but then passed the null buffer to
`memset`, causing an access violation.

For example, a client input integration might use a buffered keyboard
and check how many events are waiting without copying them. If the
player opens F8 or another MTA window at that moment, MTA owns the input
and the old proxy path could crash the client.

GTA's default keyboard and mouse devices use immediate polling, so this
requires a buffered DirectInput caller. The crash was reproduced inside
MTA using a buffered keyboard created through the real DirectInput
proxy.

<details>
<summary>Crash Stack</summary>

```text
Exception: Access violation
Module: core_d.dll

memset
CProxyDirectInputDevice8::GetDeviceData
CProxyDirectInputDevice8::GetDeviceState
```

<img width="868" height="752" alt="Crash"
src="https://github.com/user-attachments/assets/307288d8-a5e7-4c73-b7f5-7b853295bc50"
/>

</details>

## Test plan

**Runtime**

- Before Fix: A null-output query with one buffered keyboard event
caused an access violation in `memset`.
- After Fix: The same query returned `0x00000000` with `count=1`, and
the client stayed open.
- Valid Case: Queries with an output array were still cleared while the
GUI owned input.
- Harness: Normal forwarding, null-output queries, and input suppression
all passed.

**Builds and Tests**

- `Debug | Win32`: Client Core build passed, 304 client tests passed.
- `Release | Win32`: Client Core build passed, 304 client tests passed.
- Ran `clang-format`.

## Checklist

* [x] Your code should follow the [coding
guidelines](https://wiki.multitheftauto.com/index.php?title=Coding_guidelines).
* [x] Smaller pull requests are easier to review. If your pull request
is beefy, your pull request should be reviewable commit-by-commit.

Author: HeresHavi

Client/core/DXHook/CProxyDirectInputDevice8.cpp

@@ -146,7 +146,8 @@ HRESULT CProxyDirectInputDevice8::GetDeviceData(DWORD a, LPDIDEVICEOBJECTDATA b,
             }
 
             // Clear strucutre(s).
-            memset(b, 0, a * (*c));
+            if (b)
+                memset(b, 0, a * (*c));
             return hResult;
         }
     }