Fixes an edge-case where more parameters were being created than could be populated with struct field values.

Example:

type StructA struct { // maps to table_a
  TableBName string `db:"join=table_b.name"`
  TableCName string `db:"join=table_c.name"`
}

• table_a has columns table_b_id (joins to table_b) and table_c_id (joins to table_c)
• table_b has a composite key name and table_c_id (joins to table_c)

When generating statements for table_a, we shouldn't need 2 parameters for table_c_id, we just need one and we can re-use the value we got for the other column.