renderer: fixed bsp flares causing surface corruption

In R_DlightSurface, 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.

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note: discovered in my [project](https://github.com/zturtleman/spearmint) because q3 maps have BSP flares.