Validate the OID argument of pg_import_system_collations().

"SELECT pg_import_system_collations(0)" caused an assertion failure.
With a random nonzero argument --- or indeed with zero, in non-assert
builds --- it would happily make pg_collation entries with garbage
values of collnamespace.  These are harmless as far as I can tell
(unless maybe the OID happens to become used for a schema, later on?).
In any case this isn't a security issue, since the function is
superuser-only.  But it seems like a gotcha for unwary DBAs, so let's
add a check that the given OID belongs to some schema.

Back-patch to v10 where this function was introduced.

src/backend/commands/collationcmds.c

@@ -407,14 +407,16 @@ pg_import_system_collations(PG_FUNCTION_ARGS)
 	Oid			nspid = PG_GETARG_OID(0);
 	int			ncreated = 0;
 
-	/* silence compiler warning if we have no locale implementation at all */
-	(void) nspid;
-
 	if (!superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 				 errmsg("must be superuser to import system collations")));
 
+	if (!SearchSysCacheExists1(NAMESPACEOID, ObjectIdGetDatum(nspid)))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_SCHEMA),
+				 errmsg("schema with OID %u does not exist", nspid)));
+
 	/* Load collations known to libc, using "locale -a" to enumerate them */
 #ifdef READ_LOCALE_A_OUTPUT
 	{