xserver: os/log.c: ignore alternate forms in log formats

This patch fixes Xserver crashes when evdev or synaptics input drivers are used.
These (and maybe other) driver use alternate forms "%#..." in formats for
xf86IDrvLogVerb() function, which in turn uses signal-safe vpnprintf().
The last function does not recognize alternate forms and exits abnormally, which
causes Xserver to crash. The patch make vpnprintf() to ignore alternate forms.

Signed-off-by: Oleh Nykyforchyn <oleh.nyk@gmail.com>

os/log.c

@@ -410,6 +410,10 @@ vpnprintf(char *string, int size_in, const char *f, va_list args)
 
         f_idx++;
 
+        if (f[f_idx] == '#')
+        /* silently ignore alternate form */
+            f_idx++;
+
         /* silently ignore reverse justification */
         if (f[f_idx] == '-')
             f_idx++;