vga: fix invalid read after free

After calling dpy_gfx_replace_surface(s->con, surface), the outer
surface is invalid.

==5370== Invalid read of size 4
==5370==    at 0x460229: surface_bits_per_pixel (console.h:250)
==5370==    by 0x466A81: get_depth_index (vga.c:1173)
==5370==    by 0x467EC2: vga_draw_graphic (vga.c:1718)
==5370==    by 0x4687A5: vga_update_display (vga.c:1914)
==5370==    by 0x2A782E: qxl_hw_update (qxl.c:1766)
==5370==    by 0x3EB83B: graphic_hw_update (console.c:254)
==5370==    by 0x3FBE31: qemu_spice_display_refresh (spice-display.c:418)
==5370==    by 0x2A7D01: display_refresh (qxl.c:1886)
==5370==    by 0x3EEE1C: dpy_refresh (console.c:1436)
==5370==    by 0x3EB543: gui_update (console.c:192)
==5370==    by 0x3C43B3: timerlist_run_timers (qemu-timer.c:488)
==5370==    by 0x3C4416: qemu_clock_run_timers (qemu-timer.c:499)
==5370==  Address 0x22ffb1e0 is 0 bytes inside a block of size 56 free'd
==5370==    at 0x4A074C4: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5370==    by 0x4245FC: free_and_trace (vl.c:2771)
==5370==    by 0x50899AE: g_free (gmem.c:252)
==5370==    by 0x3EE8D3: qemu_free_displaysurface (console.c:1332)
==5370==    by 0x3EEDB7: dpy_gfx_replace_surface (console.c:1427)
==5370==    by 0x467EB6: vga_draw_graphic (vga.c:1714)
==5370==    by 0x4687A5: vga_update_display (vga.c:1914)
==5370==    by 0x2A782E: qxl_hw_update (qxl.c:1766)
==5370==    by 0x3EB83B: graphic_hw_update (console.c:254)
==5370==    by 0x3FBE31: qemu_spice_display_refresh (spice-display.c:418)
==5370==    by 0x2A7D01: display_refresh (qxl.c:1886)
==5370==    by 0x3EEE1C: dpy_refresh (console.c:1436)

Signed-off-by: Marc-André Lureau <marcandre.lureau@gmail.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1383664554-15248-1-git-send-email-marcandre.lureau@gmail.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
