draw: Allocate extra padding for extra shader outputs.

This prevents read buffer overflows in dup_vertex(), when draw stages
allocate extra shader outputs after the vertex buffers are allocated.

The original issue can be exercised with upcoming
piglit/tests/general/vertex-fallbacks.c test.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Cc: 21.0 21.1 <mesa-stable@lists.freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10836>

src/gallium/auxiliary/draw/draw_pipe_util.c

@@ -76,8 +76,8 @@ boolean draw_alloc_temp_verts( struct draw_stage *stage, unsigned nr )
    if (nr != 0)
    {
       unsigned i;
-      ubyte *store = (ubyte *) MALLOC( MAX_VERTEX_SIZE * nr );
-
+      ubyte *store = (ubyte *) MALLOC( MAX_VERTEX_SIZE * nr +
+                                       DRAW_EXTRA_VERTICES_PADDING );
       if (!store)
          return FALSE;