glx: init __GLXvendorInfo to NULL

Since 01ba8a8d02 the dd variable isn't necessarly initialized,
so the compiler complains:

   warning: ‘dd’ may be used uninitialized in this function

Initialize dd to NULL to fix this.

Fixes: 01ba8a8d02 ("glx: Implement GLX_EXT_no_config_context")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10418>

src/glx/g_glxglvnddispatchfuncs.c

@@ -158,7 +158,7 @@ static GLXContext dispatch_CreateContextAttribsARB(Display *dpy,
                                                    const int *attrib_list)
 {
     PFNGLXCREATECONTEXTATTRIBSARBPROC pCreateContextAttribsARB;
-    __GLXvendorInfo *dd;
+    __GLXvendorInfo *dd = NULL;
     GLXContext ret;
 
     if (config) {