ModelUnmanaged default constructor leaves members uninitialized

[RobLoach]

Problem

The default constructor of ModelUnmanaged at include/ModelUnmanaged.hpp:24 is:

ModelUnmanaged() {}

This does not initialize any of the base ::Model struct members. Fields like meshCount, materialCount, meshes, materials, meshMaterial, skeleton, currentPose, and boneMatrices are left with indeterminate values.

This causes two concrete problems:

1. Double-free / crash on destruction: If a default-constructed Model is destroyed without ever being loaded, the destructor calls UnloadModel() which tries to free the garbage pointer values in meshes, materials, etc.
2. Undefined behavior on query: Calling any getter (e.g. GetMeshCount()) on a default-constructed model returns garbage.

This is tracked in issue #351.

Suggested Implementation

Zero-initialize all members in the default constructor. The simplest approach:

ModelUnmanaged() : ::Model{} {}

This value-initializes the base struct, zeroing all scalar and pointer members. Alternatively, explicitly set each member:

ModelUnmanaged() {
    meshCount = 0;
    materialCount = 0;
    meshes = nullptr;
    materials = nullptr;
    meshMaterial = nullptr;
    transform = MatrixIdentity();
    skeleton = {0, nullptr, nullptr};
    currentPose = nullptr;
    boneMatrices = nullptr;
}

The first approach (: ::Model{}) is preferred — it's shorter and automatically covers any new fields added to raylib's Model struct in future versions.

QA

• Default-constructed ModelUnmanaged and Model can be destroyed without crash
• GetMeshCount() returns 0 on a default-constructed model
• All pointer getters return nullptr on a default-constructed model
• Existing tests continue to pass
• No regressions in move constructor/assignment (which already null out the source)