add a scary test case

tests/run-pass/transmute_fat.rs

@@ -0,0 +1,10 @@
+fn main() {
+    // If we are careful, we can exploit data layout...
+    let raw = unsafe {
+        std::mem::transmute::<&[u8], [usize; 2]>(&[42])
+    };
+    let ptr = raw[0] + raw[1];
+    let ptr = ptr as *const u8;
+    // The pointer is one-past-the end, but we decrement it into bounds before using it
+    assert_eq!(unsafe { *ptr.offset(-1) }, 42);
+}