mesa: drop unneeded assert

As seen a couple  of lines above - there's no way for the assert to
trigger.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/framebuffer.c

@@ -256,7 +256,6 @@ _mesa_reference_framebuffer_(struct gl_framebuffer **ptr,
 
       *ptr = NULL;
    }
-   assert(!*ptr);
 
    if (fb) {
       mtx_lock(&fb->Mutex);