VP8 crash fix for Xcode 5.1 / iOS7.1

https://gerrit.chromium.org/gerrit/#/c/69342/4/vp8/encoder/pickinter.c

This is absolutely serious, but I don't get what's going on here. I first thought it was a joke, but now I'm wondering why this code makes this work. Related bug: https://code.google.com/p/webrtc/issues/detail?id=3038