A... couple... of type checking bugs

I considered doing separate issues for these but I'm assuming there's some common root cause.
```c
int g() {}
int main(void)
{
int arr[1] = {NULL, 3, "lol"[arr]};
struct { int x, y, z; } s = { "oops", arr, &s, 42 };
union { int i; float f; } u;
int x = u + s;
s = g;
return &s;
}
```
Compiler is awake and sees the problem immediately.
```
% ./target/release/ccc -o a /tmp/a.c
/tmp/a.c:2:1: warning: control reaches end of non-void function [-Wreturn-type]
int g() {}
^~~
```
100% would recommend, much more exciting to use than GCC!