Be more paranoid about OOM in search_path cache.
Recent commit
f26c2368dc introduced a search_path cache, but left some
potential out-of-memory hazards. Simplify the code and make it safer
against OOM.
This change reintroduces one list_copy(), losing a small amount of the
performance gained in
f26c2368dc. A future change may optimize away
the list_copy() again if it can be done in a safer way.
Discussion: https://postgr.es/m/
e6fded24cb8a2c53d4ef069d9f69cc7baaafe9ef.camel@j-davis.com