st/mesa: Fix recurring surfaceless contexts

A current incomplete framebuffer was incorrectly used as a
st_framebuffer. When accessing st_framebuffer childs bad things happen:
e.g. st_framebuffer::iface was used to check whether its an incomplete
fb, instead we need to compare st_framebuffer::Base against
mesa_get_incomplete_framebuffer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44919
Note: This is a candidate for the 8.0 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_manager.c

@@ -688,7 +688,9 @@ st_framebuffer_reuse_or_create(struct gl_framebuffer *fb,
 {
    struct st_framebuffer *cur = st_ws_framebuffer(fb), *stfb = NULL;
 
-   if (cur && cur->iface == stfbi) {
+   /* dummy framebuffers cant be used as st_framebuffer */
+   if (cur && &cur->Base != _mesa_get_incomplete_framebuffer() &&
+       cur->iface == stfbi) {
       /* reuse the current stfb */
       st_framebuffer_reference(&stfb, cur);
    }
@@ -779,7 +781,7 @@ st_manager_flush_frontbuffer(struct st_context *st)
       return;
 
    /* never a dummy fb */
-   assert(stfb->iface);
+   assert(&stfb->Base != _mesa_get_incomplete_framebuffer());
    stfb->iface->flush_front(stfb->iface, ST_ATTACHMENT_FRONT_LEFT);
 }