Fix incorrect access to pg_index.indcollation.

Fix incorrect access to pg_index.indcollation.

Since this field is after a variable-length field, it can't simply be
accessed via the C struct for pg_index.  Fortunately, the relcache already
did the dirty work of pulling the information out to where it can be
accessed easily, so this is a one-line fix.

Andres Freund