HLSL: Using SV_ClipDistance in Geometry Shader - Crash

I have a code like this:
```
[maxvertexcount(4)]
void main(point VSO si[1], inout TriangleStream stream)
{
VSO so;
so = si[0];
for (int i = 0; i < 4; ++i)
{
... // modify position and texture coordinates for this point sprite
so.clipDistance = some_value; // float clipDistance : SV_ClipDistance;
stream.Append(so);
}
}
```
And it crashes somewhere in HlslParseContext::handleAssign -> map -> HlslParseContext::assignClipCullDistance (reading location 0x0)

Reproduce the crash with the HLSL geometry-shader example using SV_ClipDistance, then inspect HlslParseContext::handleAssign, map, and assignClipCullDistance. Trace the null access reported at that path and verify that the example is handled without crashing.

Written by the indexing model from the issue text.