hw/xen: Fix trace_xs_node_read() params

When the '--enable-trace-backends=syslog' build option is configured,
the following compilation error is encountered.

In file included from /usr/include/sys/syslog.h:207,
                 from /usr/include/syslog.h:1,
                 from ./trace/trace-hw_xen.h:224,
                 from ../hw/xen/trace.h:1,
                 from ../hw/xen/xen-bus-helper.c:13:
In function ‘syslog’,
    inlined from ‘_nocheck__trace_xs_node_read’ at ../hw/xen/trace-events:41:9,
    inlined from ‘trace_xs_node_read’ at trace/trace-hw_xen.h:903:9,
    inlined from ‘xs_node_read’ at ../hw/xen/xen-bus-helper.c:154:5:
/usr/include/bits/syslog.h:45:3: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
   45 |   __syslog_chk (__pri, __USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add a check that 'value' is not null before passing it to trace_xs_node_read().

Fixes: e6cdeee959 ("hw/xen: Add xs_node_read() helper function")
Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>
Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Message-ID: <20250523160134.218997-1-liam.merwick@oracle.com>
Signed-off-by: Anthony PERARD <anthony.perard@vates.tech>

hw/xen/xen-bus-helper.c

@@ -151,7 +151,7 @@ char *xs_node_read(struct qemu_xs_handle *h, xs_transaction_t tid,
     va_end(ap);
 
     value = qemu_xen_xs_read(h, tid, path, len);
-    trace_xs_node_read(path, value);
+    trace_xs_node_read(path, value ? value : "<null>");
     if (!value) {
         error_setg_errno(errp, errno, "failed to read from '%s'", path);
     }