draw/gs: fix up current verts in output fetching.

This was wrong since I added multi-stream support in
draw/gs: track emitted prims + verts per stream

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5555>

src/gallium/auxiliary/draw/draw_gs.c

@@ -336,7 +336,7 @@ llvm_fetch_gs_outputs(struct draw_geometry_shader *shader,
 
    output_ptr += shader->stream[stream].emitted_vertices * shader->vertex_size;
    for (i = 0; i < shader->vector_length - 1; ++i) {
-      int current_verts = shader->llvm_emitted_vertices[i];
+      int current_verts = shader->llvm_emitted_vertices[i + (stream * shader->vector_length)];
       int next_verts = shader->llvm_emitted_vertices[i + 1 + (stream * shader->vector_length)];
 #if 0
       int j;