mesa/st: draw_vbo: initialize restart_index too

mesa/st: draw_vbo: initialize restart_index too

restart_index is later always used in a comparison, so it should be
initialized properly.

Fixes valgrind warning:
 Conditional jump or move depends on uninitialised value(s)
    at 0xB8D682F: r600_draw_vbo (r600_state_common.c:2153)
    by 0xB71F743: u_vbuf_draw_vbo (u_vbuf.c:1156)
    by 0xB3B92DB: st_draw_vbo (st_draw.c:235)
    by 0xB36B1AE: vbo_draw_arrays (vbo_exec_array.c:391)
    by 0xB36BB0D: vbo_exec_DrawArrays (vbo_exec_array.c:550)
    by 0x10A989: piglit_display (textureSize.c:157)
    by 0x4F8F174: run_test (piglit_fbo_framework.c:52)
    by 0x4F7BA12: piglit_gl_test_run (piglit-framework-gl.c:229)
    by 0x10A60A: main (textureSize.c:71)
 Uninitialised value was created by a stack allocation
    at 0xB3B90B0: st_draw_vbo (st_draw.c:143)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>