sql: assign collation to indexes in CREATE TABLE

Before this patch, if an index was created due to a column's UNIQUE
constraint or a column's PRIMARY KEY constraint before adding a
collation, and if the column's fieldno was not equal to the index's
position in space->index, the collation would not be assigned to the
index.

Also, this patch fixes an assertion in debug build for the case when an
index with more that one field was created before a collation was added.

Closes #9229

NO_DOC=bugfix