explain what happens in a test

tests/run-pass/transmute_fat2.rs

@@ -8,5 +8,6 @@ fn main() {
     let bad = unsafe {
         std::mem::transmute::<u64, &[u8]>(42)
     };
+    // This created a slice with length 0, so the following will fail the bounds check.
     bad[0];
 }

tests/run-pass/transmute_fat2.stderr

@@ -1 +1 @@
-thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', $DIR/transmute_fat2.rs:11:5
+thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', $DIR/transmute_fat2.rs:12:5