Attempt to fix the segfaulting osx bots

Attempt to fix the segfaulting osx bots

Upon inspecting the core dumps, they're all segfaulting at the same instruction
with the same value in a register that looks fishy. It appears to be indexing
into an array with a -1 index and then getting some weird overflow and dying.

I have attempted to fix this as part of
alexcrichton/libuv@fd5308383c575472edb2163d823dc6670bf59609,
but I am unsure of whether this is the actual cause of the problem, so I am not
going to upstream it just yet. I have a fairly high confidence that this is
indeed the problem, but I want to make sure that the bots to segfault all over
the place before upstreaming.