Call Variant destructor in operator= to prevent a memory leak

[BimDav]

This is to prevent memory leaks in the present implementation of Variant operator= in GDNative (see #647, #643)

Bugsquad edit:

• Fixes godot-cpp Variant::operator= Leaks memory #643
• Fixes Putting Transforms into Array leaks memory #647

[BimDav]

This approach was preferred to the one started in #648 because it does not need a change in godot-headers and other operator= function in godot-cpp are implemented this way.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.