xserver: fix an off-by-one error that lead to asprintf("%s", NULL)

reported by deraadt@, fix reviewed in X.Org by Keith Packard.

xserver/hw/xfree86/common/xf86AutoConfig.c

@@ -395,8 +395,7 @@ autoConfigDevice(GDevPtr preconf_device)
 
             /* for each other driver found, copy the first screen, insert it
              * into the list of screens and set the driver */
-            i = 0;
-            while (i++ < num_matches) {
+            for (i = 1; i < num_matches; i++) {
                 if (!copyScreen(slp[0].screen, ptr, i, matches[i]))
                     return NULL;
             }