glsl-in: Fix panic when culling nested block

[JCapucho]

Under certatin circumstances the parser would panic because of culling
using an index out of bound, for example take the following code:

{
    x = 1; // Index 0 on nested block
    discard; // Index 1 on nested block
} // Index 0 on block

The parser seeing this would attempt to cull the block using the start
index of 1 which would be out of bound and panic. Now instead the nested
block properly does it's internal culling and the outer block culls
everything after the nested block only.