Calls to __wt_debug_tree_all to dump a LAS file from GDB can cause segfault

When looking to dump the WiredTigerLAS.wt file from within GDB using __wt_debug_tree_all, it is possible to get into a state where we are dumping a file from a session which has no dhandle set. This is commonly seen when dumping the WiredTigerLAS.wt file from within the sweep server.