os: Fix regression with FatalError not calling va_start

Regression From: a818b30598

Found-by: Colin Harrison <colin.harrison@virgin.net>
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>

os/log.c

@@ -601,6 +601,8 @@ FatalError(const char *f, ...)
     else
         ErrorF("\nFatal server error:\n");
 
+    va_start(args, f);
+
     /* Make a copy for OsVendorFatalError */
     va_copy(args2, args);