misc: Fix a small pile of warnings found with diagnostics turned way up

* possible null pointer dereferences
 * non-annotated fallthroughs in case statements

Change-Id: I2332e0e0a4203ceca0999291ead112b8ab94c613

apps/plugins/goban/sgf_storage.c

@@ -152,6 +152,7 @@ free_tree_sgf (void)
     {
         rb->splash (5 * HZ,
                     "Error allocating first node!  Please exit immediately.");
+        return;
     }
 
     get_node (tree_head)->props = NO_PROP;
@@ -498,4 +499,3 @@ get_prop (int handle)
     }
     return &(storage_buffer[index][handle].prop);
 }
-