[PATCH] Fix flakey autobuilds due to DNS tests

[Garming Sam]

Hi,

I've finally managed to figure out the cause of the flapping DNS tests
thanks to a changeset which caused them to trigger much more often. This
was due to dns_name_equal failing to correctly compare zero length
strings. Such a case occurs in the DNS tests where a DNS name was set to
'.' and then subsequently transformed to ''.

dns_name_equal accidentally compared the index at -1 to '.' and so every
now and again they would not be equal when comparing two zero length
strings. Attached is a patch to prevent the out of bounds read and
prevent any more flapping DNS tests.

Please review and push.


Cheers,

Garming

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