[PATCH] os: Make sure big requests have sufficient length.

[Michal Srb]

On pondělí 9. října 2017 17:14:44 CEST Eric Anholt 
> I tried the updated testcase and that didn't crash for me, either.  My
> v2 (which I've now sent out) testcase times out in 30 seconds without
> the fix and passes with the fix.  I'd love your review if you like that
> as a solution.

PolyLine is not crashing because it has `if (npoint > 1)` test and the 
`npoint` (int) overflowed into negative numbers.

If you change the XCB_POLY_LINE into XCB_POLY_RECTANGLE in your test it is 
enough to crash X server on my machine.

Michal Srb