[TODO] Remove C assert

[syoyo]

Describe the issue

tinygltf has some C assert, which is not recommended to run tinygltf in secure environment.
(e.g. read malcious glTF file in a server)

The code should not segfault and errors should be reported properly.
Removing assert recommended.

To Reproduce

• OS : Any
• Compiler, compiler version, compile options: Any
• Please attach minimal and reproducible .glTF file if you got an issue related to .glTF data : N/A

Expected behaviour

No assert in tiny_gltf.h.
(Raise an C++ exception when TINYGLTF_NOEXCEPTION is not set(an app is compiled with C++ exception enabled))

Screenshots

N/A

Additional context

N/A

[syoyo]

Prepared remove-assert branch: https://github.com/syoyo/tinygltf/tree/remove-assert