cirrus_vga: fix division by 0 for color expansion rop

Commit d85d0d38 introduces a regression
with Windows ME that leads to a division by 0 and a crash.

It uses the color expansion rop with the source pitch set to 0. This is
something allowed, as the manual explicitely says "When the source of
color-expand data is display memory, the source pitch is ignored.".

This patch fixes this regression by computing sx, sy and others
variables only if they are going to be used later, that is for a plain
copy ROP. It basically consists in moving code.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/cirrus_vga.c

@@ -675,10 +675,15 @@ static void cirrus_do_copy(CirrusVGAState *s, int dst, int src, int w, int h)
 {
     int sx, sy;
     int dx, dy;
-    int width, height;
     int depth;
     int notify = 0;
 
+    /* make sure to only copy if it's a plain copy ROP */
+    if (*s->cirrus_rop == cirrus_bitblt_rop_fwd_src ||
+        *s->cirrus_rop == cirrus_bitblt_rop_bkwd_src) {
+
+        int width, height;
+
         depth = s->vga.get_bpp(&s->vga) / 8;
         s->vga.get_resolution(&s->vga, &width, &height);
 
@@ -707,11 +712,7 @@ static void cirrus_do_copy(CirrusVGAState *s, int dst, int src, int w, int h)
             (dx + w) <= width && (dy + h) <= height) {
             notify = 1;
         }
-
-    /* make to sure only copy if it's a plain copy ROP */
-    if (*s->cirrus_rop != cirrus_bitblt_rop_fwd_src &&
-	*s->cirrus_rop != cirrus_bitblt_rop_bkwd_src)
-	notify = 0;
+    }
 
     /* we have to flush all pending changes so that the copy
        is generated at the appropriate moment in time */