binder_common: Fix uninitialized marshalling

C# uses `long`s to access many native values. With `PtrToArg<m_enum>` and
`PtrToArg<bitfield<m_enum>>` this isn't a problem, as C++ code converts
through a `*(int64_t*)` cast in assignment, so all 64-bits are initialized.

However, with `PtrToArg<char32_t>`, value assignment happens through an
`*(int *)` cast, leaving 32 bits uninitialized where `int` is 32 bits. On
platforms where `int` is 16 bits, there are presumably 48 bits uninitialized,
though there are very few platforms where this is still the case.

The easiest way to see the practical effects of this is by looking at
`EventInputKey.Unicode`:

```csharp
    public override void _Input(InputEvent @event) {
        if (@event is InputEventKey keyEvent) {
            if (keyEvent.IsPressed() && !keyEvent.Echo) {
                var raw = keyEvent.Unicode;
                var value = raw & 0xffffffff;
                GD.Print($"Key pressed: raw: {raw}; masked: {(char) value} ({value})");
            }
        }
    }
```

Pressing 'a' emits the following line:
```
Key pressed: raw: -3617008645356650399; masked: a (97)
```

Examining execution flow in gdb shows this conversion going through the
following line:
```
PtrToArg<char32_t>::encode (p_ptr=0x7ffcd5bb4b18, p_val=97 U'a') at ./core/variant/binder_common.h:221
221			*(int *)p_ptr = p_val;
```

Here, `p_val` is still 97, which is the value `InputEventKey.Unicode`
is expected to have. After assignment, `p *(int64_t *)0x7ffcd5bb4b18` displays
`-3617008645356650399`, with only the lower 32 bits being properly assigned,
and is the value we see from C#.

With this patch applied, the above testing `_Input` now prints:
```
Key pressed: raw: 97; masked: a (97)
```

Thank you to blujay1269 for asking about an unexpected value they saw in
`EventInputKey.Unicode`, which prompted this investigation.

core/variant/binder_common.h

@@ -214,11 +214,11 @@ struct VariantCaster<char32_t> {
 template <>
 struct PtrToArg<char32_t> {
 	_FORCE_INLINE_ static char32_t convert(const void *p_ptr) {
-		return char32_t(*reinterpret_cast<const int *>(p_ptr));
+		return char32_t(*reinterpret_cast<const int64_t *>(p_ptr));
 	}
 	typedef int64_t EncodeT;
 	_FORCE_INLINE_ static void encode(char32_t p_val, const void *p_ptr) {
-		*(int *)p_ptr = p_val;
+		*(int64_t *)p_ptr = p_val;
 	}
 };