Some connectors (JDBC, Node.js) can sent non-default collation IDs
in the handshake packet. The code in thd_init_client_charset() handling
@@character_set_collations did not expect that and crashed on DBUG_ASSERT.

Changing the code to ignore @@character_set_collations in case of non-default
IDs. This fixes the problem in a backward compatible
(with pre-@@character_set_collations server versions) way for such connectors
sending non-default IDs.