rebind-fn.rs crashes with RUST_CC_ZEAL=1

[jruderman]

src/test/run-pass/rebind-fn.rs crashes when run with RUST_CC_ZEAL=1.

[catamorphism]

Currently results in Assertion failed: (box->ref_count >= 1), function box_body_td, file ./src/rt/rust_shape.h, line 704. -- same failure as #1466

[nikomatsakis]

I believe this is now fixed.