handle zero-size allocations correctly

The `malloc` family of functions may return a null pointer for a
zero-size allocation, which should not be interpreted as an
out-of-memory error.

If the implementation does not return a null pointer, then handling
this will result in memory savings for zero-size types.

This also switches some code to `malloc_raw` in order to maintain a
centralized point for handling out-of-memory in `rt::global_heap`.

Closes #11634

src/libnative/io/file.rs

@@ -13,7 +13,7 @@
 use std::c_str::CString;
 use std::io::IoError;
 use std::io;
-use std::libc::c_int;
+use std::libc::{c_int, c_void};
 use std::libc;
 use std::os;
 use std::rt::rtio;
@@ -548,7 +548,7 @@ pub fn readdir(p: &CString) -> IoResult<~[Path]> {
             let p = Path::new(p);
             let star = p.join("*");
             as_utf16_p(star.as_str().unwrap(), |path_ptr| {
-                let wfd_ptr = malloc_raw(rust_list_dir_wfd_size() as uint);
+                let wfd_ptr = malloc_raw(rust_list_dir_wfd_size() as uint) as *c_void;
                 let find_handle = FindFirstFileW(path_ptr, wfd_ptr as HANDLE);
                 if find_handle as libc::c_int != INVALID_HANDLE_VALUE {
                     let mut paths = ~[];