glx: proposed fix for setSwapInterval

When mesa gets a DRI2 1.1 connection (as experienced with
vmwware DDX) we don't get a pointer for this.

Don't explode just keep going.

Fixes: 60ebeb4608 ("glx: Implement GLX_EXT_swap_control for DRI2 and DRI3")
Reviewed-by: Adam Jackson <ajaX@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9184>

src/glx/glxcmds.c

@@ -1863,8 +1863,8 @@ glXSwapIntervalEXT(Display *dpy, GLXDrawable drawable, int interval)
       __glXSendError(dpy, BadValue, interval, 0, True);
       return;
    }
-
-   pdraw->psc->driScreen->setSwapInterval(pdraw, interval);
+   if (pdraw->psc->driScreen->setSwapInterval)
+      pdraw->psc->driScreen->setSwapInterval(pdraw, interval);
 #endif
 }