Fix null dereference when building a texture from memory

[hard25670559]

Problem

r_tex_c(r_ITexManager*, std::unique_ptr<image_c>, int) assigns the mip set to its parameter rather than to the member PerformUpload reads:

r_tex_c::r_tex_c(r_ITexManager* manager, std::unique_ptr<image_c> img, int flags)
{
    Init(manager, {}, flags);

    // Direct upload
    img = BuildMipSet(std::move(img));   // <- parameter, not the member
    PerformUpload(this);                 // <- reads tex->img, still null
}

PerformUpload then does tex->Upload(*tex->img, tex->flags) on a null img, so every use of this constructor crashes. There appear to be no in-tree callers, which is presumably why it has gone unnoticed.

Change

Assign to the member.

Verification

Found while building textures from memory in a downstream branch: the constructor crashed with EXC_BAD_ACCESS at r_tex_c::Upload on the first call, and worked after this one-line change. Existing texture loading paths go through LoadFile/Upload directly and are untouched.