vnc: fix qemu crash because of SIGSEGV

The backtrace is:

0x00007f0b75cdf880 in pixman_image_get_stride () from /lib64/libpixman-1.so.0
0x00007f0b77bcb3cf in vnc_server_fb_stride (vd=0x7f0b7a1a2bb0) at ui/vnc.c:680
vnc_dpy_copy (dcl=0x7f0b7a1a2c00, src_x=224, src_y=263, dst_x=319, dst_y=363, w=1, h=1) at ui/vnc.c:915
0x00007f0b77bbcc35 in dpy_gfx_copy (con=0x7f0b7a146210, src_x=src_x@entry=224, src_y=src_y@entry=263, dst_x=dst_x@entry=319,
dst_y=dst_y@entry=363, w=1, h=1) at ui/console.c:1575
0x00007f0b77bbda4e in qemu_console_copy (con=<optimized out>, src_x=src_x@entry=224, src_y=src_y@entry=263, dst_x=dst_x@entry=319,
dst_y=dst_y@entry=363, w=<optimized out>, h=<optimized out>) at ui/console.c:2111
0x00007f0b77ac0980 in cirrus_do_copy (h=<optimized out>, w=<optimized out>, src=<optimized out>, dst=<optimized out>, s=0x7f0b7b086090) at hw/display/cirrus_vga.c:774
cirrus_bitblt_videotovideo_copy (s=0x7f0b7b086090) at hw/display/cirrus_vga.c:793
cirrus_bitblt_videotovideo (s=0x7f0b7b086090) at hw/display/cirrus_vga.c:915
cirrus_bitblt_start (s=0x7f0b7b086090) at hw/display/cirrus_vga.c:1056
0x00007f0b77965cfb in memory_region_write_accessor (mr=0x7f0b7b096e40, addr=320, value=<optimized out>, size=1, shift=<optimized out>,mask=<optimized out>, attrs=...) at /root/rpmbuild/BUILD/master/qemu/memory.c:525
0x00007f0b77963f59 in access_with_adjusted_size (addr=addr@entry=320, value=value@entry=0x7f0b69a268d8, size=size@entry=4,
access_size_min=<optimized out>, access_size_max=<optimized out>, access=access@entry=0x7f0b77965c80 <memory_region_write_accessor>,
mr=mr@entry=0x7f0b7b096e40, attrs=attrs@entry=...) at /root/rpmbuild/BUILD/master/qemu/memory.c:591
0x00007f0b77968315 in memory_region_dispatch_write (mr=mr@entry=0x7f0b7b096e40, addr=addr@entry=320, data=18446744073709551362,
size=size@entry=4, attrs=attrs@entry=...) at /root/rpmbuild/BUILD/master/qemu/memory.c:1262
0x00007f0b779256a9 in address_space_write_continue (mr=0x7f0b7b096e40, l=4, addr1=320, len=4, buf=0x7f0b77713028 "\002\377\377\377",
attrs=..., addr=4273930560, as=0x7f0b7827d280 <address_space_memory>) at /root/rpmbuild/BUILD/master/qemu/exec.c:2544
address_space_write (as=<optimized out>, addr=<optimized out>, attrs=..., buf=<optimized out>, len=<optimized out>) at /root/rpmbuild/BUILD/master/qemu/exec.c:2601
0x00007f0b77925c1d in address_space_rw (as=<optimized out>, addr=<optimized out>, attrs=..., attrs@entry=...,
buf=buf@entry=0x7f0b77713028 "\002\377\377\377", len=<optimized out>, is_write=<optimized out>) at /root/rpmbuild/BUILD/master/qemu/exec.c:2703
0x00007f0b77962f53 in kvm_cpu_exec (cpu=cpu@entry=0x7f0b79fcc2d0) at /root/rpmbuild/BUILD/master/qemu/kvm-all.c:1965
0x00007f0b77950cc6 in qemu_kvm_cpu_thread_fn (arg=0x7f0b79fcc2d0) at /root/rpmbuild/BUILD/master/qemu/cpus.c:1078
0x00007f0b744b3dc5 in start_thread (arg=0x7f0b69a27700) at pthread_create.c:308
0x00007f0b70d3d66d in clone () from /lib64/libc.so.6

The code path while meeting segfault:
 vnc_dpy_copy
   vnc_update_client
     vnc_disconnect_finish [while vnc_disconnect_start() is invoked because somethins wrong]
       vnc_update_server_surface
         vd->server = NULL;
   vnc_server_fb_stride
     pixman_image_get_stride(vd->server)

Let's add a non-NULL check before calling vnc_server_fb_stride() to avoid segmentation fault.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel P. Berrange <berrange@redhat.com>
Reported-by: Yanying Zhuang <ann.zhuangyanying@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1472788698-120964-1-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
