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 9fbc3f31.

Author: Jaime Casanova
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAJKUy5gF4=_=qhJ1VX_tSGFfjKHb9BvzhRYWSApJD=Bfwp2SBw@mail.gmail.com

contrib/pg_stat_statements/pg_stat_statements.c

@@ -1898,6 +1898,11 @@ pg_stat_statements_info(PG_FUNCTION_ARGS)
 	Datum		values[PG_STAT_STATEMENTS_INFO_COLS];
 	bool		nulls[PG_STAT_STATEMENTS_INFO_COLS];
 
+	if (!pgss || !pgss_hash)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("pg_stat_statements must be loaded via shared_preload_libraries")));
+
 	/* Build a tuple descriptor for our result type */
 	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
 		elog(ERROR, "return type must be a row type");