xf86XvMCScreenInit: Clear pScreenPriv->dixinfo when freeing pAdapt

Fixes double-free later in xf86XvMCCloseScreen, which would generally
cause fireworks.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>

hw/xfree86/common/xf86xvmc.c

@@ -187,6 +187,7 @@ xf86XvMCScreenInit(ScreenPtr pScreen,
         }
         if (!pAdapt[i].xv_adaptor) {
             /* no adaptor by that name */
+            pScreenPriv->dixinfo = FALSE;
             free(pAdapt);
             return FALSE;
         }