xfree86: silence warning: ‘ramplen’ may be used uninitialized

'ramplen' is only set and used if stuff->size > 0 but the compiler doesn't
pick that up.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

hw/xfree86/dixmods/extmod/xf86vmode.c

@@ -1540,7 +1540,7 @@ ProcXF86VidModeGetGammaRamp(ClientPtr client)
 {
     CARD16 *ramp = NULL;
     int n, length;
-    size_t ramplen;
+    size_t ramplen = 0;
     xXF86VidModeGetGammaRampReply rep;
     REQUEST(xXF86VidModeGetGammaRampReq);