st/mesa: Fix topogun-1.06-orc-84k-resize.trace crash

We need to initialize all fields in rs->prim explicitly while
creating new rastpos stage.

Fixes: bac8534267 ("st/mesa: allow glDrawElements to work with GL_SELECT
feedback")

v2: Initializing all fields in rs->prim as per Ilia.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/mesa/state_tracker/st_cb_rasterpos.c

@@ -208,6 +208,10 @@ new_draw_rastpos_stage(struct gl_context *ctx, struct draw_context *draw)
    rs->prim.end = 1;
    rs->prim.start = 0;
    rs->prim.count = 1;
+   rs->prim.pad = 0;
+   rs->prim.num_instances = 1;
+   rs->prim.base_instance = 0;
+   rs->prim.is_indirect = 0;
 
    return rs;
 }