BIND 10 #567: RdataIterator::getCurrent() should throw if invalid.

[BIND 10 Development]

#567: RdataIterator::getCurrent() should throw if invalid.
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           Reporter:  vorner         |                      Owner:
               Type:  enhancement    |                     Status:  new
           Priority:  minor          |                  Milestone:  A-Team-
          Component:  Unclassified   |  Task-Backlog
          Sensitive:  0              |                   Keywords:
Add Hours to Ticket:  0              |  Estimated Number of Hours:  0
        Total Hours:  0              |                  Billable?:  1
                                     |                  Internal?:  0
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 When the RdataIterator points at the last entry and getCurrent is called,
 it should throw an exception.

 This way, we can just use the first Rdata in RRset and don't need to care
 about empty RRsets (in case of DNAME, for example, empty DNAME rrset is
 wrong and the code makes no sense).

 After this is finished, the assert in CNAME synthesis code for DNAME in
 auth/query.cc should be removed (see #505).

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Ticket URL: <https://bind10.isc.org/ticket/567>
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