Autoconfig: Fix the loop adding dectected drivers to the list of screens

This loop was written in a buggy style, causing a NULL driver ptr to be
passed to copyScreen(). copyScreen() only uses that to generate an
identifier string, so this is mostly harmless on systems that accept
NULL for asprintf() "%s" format. (the generated identifiers are off
by one wrt the driver names and the last one contains NULL.

For systems that don't accept NULL for '%s' this would cause a
segmentation fault when this code is used (no xorg.conf, but partial
config in xorg.conf.d for instance).

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
Reviewed-by: Keith Packard <keithp@keithp.com>

hw/xfree86/common/xf86AutoConfig.c

@@ -389,8 +389,7 @@ 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;
             }