Fix SIGFPE race for vnc display resize

cc39a92cbfc8 fixed a SIGFPE where the screen is resized to width/hight 1 and then receives a mouse click. However, there is a still a tiny window here for a race between the test for width/height > 1 and the division.

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Sending this as I've just seen a SIGFPE from one of the qemu-kvm VMs running in our public-facing cluster. Running gdb on the resulting core dump pointed at line #1424 of vnc.c:

1423 if (vs->absolute) {
1424 kbd_mouse_event(ds_get_width(vs->ds) > 1 ?
1425 x * 0x7FFF / (ds_get_width(vs->ds) - 1) : 0x4000,
1426 ds_get_height(vs->ds) > 1 ?
1427 y * 0x7FFF / (ds_get_height(vs->ds) - 1) : 0x4000,
1428 dz, buttons);

I think this must have been a (tight) race between the comparison and the division. This should probably go to the 0.12-stable branch too as that's where I saw the crash. I can send a rebased patch if that's more convenient?

A pity this crash happened just after the release of 0.12.5 rather than a week or two earlier!

 ui/vnc.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

index 7fc40ac..e04ebdf 100644
@@ -1410,10 +1410,10 @@ static void pointer_event(VncState *vs, int button_mask,
         dz = 1;
 
     if (vs->absolute) {
-        kbd_mouse_event(ds_get_width(vs->ds) > 1 ?
-                          x * 0x7FFF / (ds_get_width(vs->ds) - 1) : 0x4000,
-                        ds_get_height(vs->ds) > 1 ?
-                          y * 0x7FFF / (ds_get_height(vs->ds) - 1) : 0x4000,
+        int width = ds_get_width(vs->ds);
+        int height = ds_get_height(vs->ds);
+        kbd_mouse_event(width > 1 ? x * 0x7FFF / (width - 1) : 0x4000,
+                        height > 1 ? y * 0x7FFF / (height - 1) : 0x4000,
                         dz, buttons);
     } else if (vnc_has_feature(vs, VNC_FEATURE_POINTER_TYPE_CHANGE)) {
         x -= 0x7FFF;

1.7.1.1