cirrus: don't overflow CirrusVGAState->cirrus_bltbuf

This is CVE-2014-8106.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/display/cirrus_vga.c

@@ -293,6 +293,10 @@ static bool blit_is_unsafe(struct CirrusVGAState *s)
     assert(s->cirrus_blt_width > 0);
     assert(s->cirrus_blt_height > 0);
 
+    if (s->cirrus_blt_width > CIRRUS_BLTBUFSIZE) {
+        return true;
+    }
+
     if (blit_region_is_unsafe(s, s->cirrus_blt_dstpitch,
                               s->cirrus_blt_dstaddr & s->cirrus_addr_mask)) {
         return true;