st/mesa: fix unconditional return in st_framebuffer_iface_remove

Noticed by James Legg @ Feral.

Cc: 17.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/mesa/state_tracker/st_manager.c

@@ -560,7 +560,7 @@ st_framebuffer_iface_remove(struct st_manager *smapi,
       (struct st_manager_private *)smapi->st_manager_private;
    struct hash_entry *entry;
 
-   if (!smPriv || !smPriv->stfbi_ht);
+   if (!smPriv || !smPriv->stfbi_ht)
       return;
 
    mtx_lock(&smPriv->st_mutex);