glx: fix uninitialized var in __glXDRIscreenProbe

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=59825Signed-off-by: Piotr Dziwinski <piotrdz@gmail.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

glx/glxdri.c

@@ -971,6 +971,8 @@ __glXDRIscreenProbe(ScreenPtr pScreen)
     size_t buffer_size;
     ScrnInfoPtr pScrn = xf86ScreenToScrn(pScreen);
 
+    framebuffer.base = NULL;
+
     if (!xf86LoaderCheckSymbol("DRIQueryDirectRenderingCapable") ||
         !DRIQueryDirectRenderingCapable(pScreen, &isCapable) || !isCapable) {
         LogMessage(X_INFO,