Add a test for that last bug. Oops.

Author: msullivan

src/test/run-fail/small-negative-indexing.rs

@@ -0,0 +1,6 @@
+
+fn main() {
+    let v = vec::from_fn(1024u) {|n| n};
+    // this should trip a bounds check
+    log(error, v[-1i8]);
+}