warning: indirection of non-volatile null pointer will be deleted, not trap

STR:
```c
#include

int main()
{
printf("Data at address zero: %x\n", *(int*)0);
}
```

```
E:\code\tgifelse>emcc a.c -o a.js
a.c:5:39: warning: indirection of non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
printf("Data at address zero: %x\n", *(int*)0);
^~~~~~~~
a.c:5:39: note: consider using __builtin_trap() or qualifying pointer with 'volatile'
1 warning generated.

E:\code\tgifelse>node a.js
Data at address zero: 63736d65
```

This warning looks odd, because the memory load did not get deleted like the warning threatened to do.

The program behaved like I expected it to: load and print the u32 at address zero, which contains the magic bytes for Emscripten stack check.

I wonder if that LLVM warning message should somehow be adjusted for Wasm? Maybe `"warning: indirection of non-volatile null pointer will actually (attempt to) load from memory address zero, and not trap"`?

(We do already have the `-s SAFE_HEAP=1` build mode that will add the necessary checks for this for correctness checks)