test: Use vec::as_imm_buf in bug-2470-bounds-check-overflow.rs

src/test/run-fail/bug-2470-bounds-check-overflow.rs

@@ -9,7 +9,7 @@ fn main() {
     // huge).
 
     let x = ~[1u,2u,3u];
-    do vec::as_buf(x) |p, _len| {
+    do vec::as_imm_buf(x) |p, _len| {
         let base = p as uint;                     // base = 0x1230 say
         let idx = base / sys::size_of::<uint>();  // idx  = 0x0246 say
         error!("ov1 base = 0x%x", base);
@@ -21,4 +21,4 @@ fn main() {
         // This should fail.
         error!("ov1 0x%x",  x[idx]);
     }
-}
+}