glx: replace assertion with conditional

See fd.o bug 26832.

src/glx/glx_pbuffer.c

@@ -379,8 +379,8 @@ CreateDrawable(Display * dpy, const __GLcontextModes * fbconfig,
    req->glxwindow = (GLXWindow) XAllocID(dpy);
    req->numAttribs = (CARD32) i;
 
-   assert(attrib_list);
-   memcpy(data, attrib_list, 8 * i);
+   if (attrib_list)
+      memcpy(data, attrib_list, 8 * i);
 
    UnlockDisplay(dpy);
    SyncHandle();