st/mesa: handle stfbi being NULL on entry of st_framebuffer_reuse_or_create

Apparently this can happen. Just bail out early in that case, as all the called
functions return NULL in that case.

Fixes weston-terminal for me.

Fixes: 147d7fb772 ("st/mesa: add a winsys buffers list in st_context")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>

src/mesa/state_tracker/st_manager.c

@@ -788,6 +788,9 @@ st_framebuffer_reuse_or_create(struct st_context *st,
 {
    struct st_framebuffer *cur = NULL, *stfb = NULL;
 
+   if (!stfbi)
+	return NULL;
+
    /* Check if there is already a framebuffer object for the specified
     * framebuffer interface in this context. If there is one, use it.
     */