allocated four times the polys for buffers in bsp_clip_*

Otherwise we'll blow past the buffer in the created polygon lists.
We can produce twice as many polygons as we consume if every one of them
gets split, so the list was overflowing in such a case into memory that wasn't
its.
Merge pull request #46 from sshirokov/slava-sucks-at-memory

Allocate four times the polys for buffers in bsp_clip_*