Fix getting/setting huge env vars on windows

On windows, the GetEnvironmentVariable function will return the necessary buffer
size if the buffer provided was too small. This case previously fell through the
checks inside of fill_utf16_buf_and_decode, tripping an assertion in the `slice`
method.

This adds an extra case for when the return value is >= the buffer size, in
which case we assume the return value as the new buffer size and try again.

Closes #12376

Author: alexcrichton

src/libstd/os.rs

@@ -113,13 +113,15 @@ pub mod win32 {
             let mut done = false;
             while !done {
                 let mut buf = vec::from_elem(n as uint, 0u16);
-                let k = f(buf.as_mut_ptr(), TMPBUF_SZ as DWORD);
+                let k = f(buf.as_mut_ptr(), n);
                 if k == (0 as DWORD) {
                     done = true;
                 } else if k == n &&
                           libc::GetLastError() ==
                           libc::ERROR_INSUFFICIENT_BUFFER as DWORD {
                     n *= (2 as DWORD);
+                } else if k >= n {
+                    n = k;
                 } else {
                     done = true;
                 }
@@ -1494,6 +1496,16 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_env_set_get_huge() {
+        let n = make_rand_name();
+        let s = "x".repeat(10000);
+        setenv(n, s);
+        assert_eq!(getenv(n), Some(s));
+        unsetenv(n);
+        assert_eq!(getenv(n), None);
+    }
+
     #[test]
     fn test_env_setenv() {
         let n = make_rand_name();