Fix crash of pg_stat_statements_info() without library loaded
Other code paths are already protected against this case, and _PG_init()
warns about that in pg_stat_statements.c. While on it, I have checked
the other extensions of the tree but did not notice any holes.
Oversight in
9fbc3f3.
Author: Jaime Casanova
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAJKUy5gF4=_=qhJ1VX_tSGFfjKHb9BvzhRYWSApJD=Bfwp2SBw@mail.gmail.com