protect again arbitrarily large window size

Observed with nginx with SPDY 3.1 patch applied:

http://mailman.nginx.org/pipermail/nginx-devel/2014-January/004890.html

2014-01-28 19:08:18.230 eBay[55680:720b] SPDY [DEBUG] received
WINDOW_UPDATE.0 (+2147418111)

This causes a very large NSData allocation, and an immediate crash.