Add xfail'd test for rust-lang#6762.

src/test/compile-fail/issue-6762.rs

@@ -0,0 +1,24 @@
+//xfail-test
+
+// Creating a stack closure which references an owned pointer and then
+// transferring ownership of the owned box before invoking the stack
+// closure results in a crash.
+
+fn twice(x: ~uint) -> uint
+{
+     *x * 2
+}
+
+fn invoke(f : &fn() -> uint)
+{
+     f();
+}
+
+fn main()
+{
+      let x  : ~uint         = ~9;
+      let sq : &fn() -> uint = || { *x * *x };
+
+      twice(x);
+      invoke(sq);
+}