draw: don't crash if GS doesn't emit anything

Technically it's legal for geometry shader to not emit any
vertices. It's silly, but perfectly legal, so lets make draw
stop crashing if it happens.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline.c

@@ -299,6 +299,15 @@ static void fetch_pipeline_generic( struct draw_pt_middle_end *middle,
          }
       }
    }
+   if (prim_info->count == 0) {
+      debug_printf("GS/IA didn't emit any vertices!\n");
+      
+      FREE(vert_info->verts);
+      if (free_prim_info) {
+         FREE(prim_info->primitive_lengths);
+      }
+      return;
+   }
 
 
    /* Stream output needs to be done before clipping.

src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline_llvm.c

@@ -397,6 +397,15 @@ llvm_pipeline_generic( struct draw_pt_middle_end *middle,
          }
       }
    }
+   if (prim_info->count == 0) {
+      debug_printf("GS/IA didn't emit any vertices!\n");
+      
+      FREE(vert_info->verts);
+      if (free_prim_info) {
+         FREE(prim_info->primitive_lengths);
+      }
+      return;
+   }
 
    /* stream output needs to be done before clipping */
    draw_pt_so_emit( fpme->so_emit, vert_info, prim_info );