Undeclared identifier as function parameter causes null dereference.

If the shader source has an undeclared identifier used as a parameter to a function expecting a struct, [glsTType::sameStructType.](https://github.com/KhronosGroup/glslang/blob/master/glslang/Include/Types.h#L2368) will dereference a null pointer.
```
#version 460 core

struct S {
float f;
};

void Fun(in S a)
{
}

void main()
{
Fun(a);
}
```