renderer: fixed bsp flares causing surface corruption

gen isn't pointing to a srfGeneric_t if not SF_FACE, SF_TRIANGLES, SF_GRID, or SF_FOLIAGE.
Writing to it causes unknown memory to be overwritten.

Default ET maps do not have BSP flares (SF_FLARE), which use the affected code.
Merge pull request #55 from zturtleman/bsp_flares

renderer: fixed bsp flares causing surface corruption