dns_srv: Fix SRV sorting when records with priority zero exist with non-zero.

The DNS SRV sorting code currently has an issue when records with a priority
of zero exist with records of a non-zero priority. This occurs because the
sorting code considers zero to mean unset when in reality is a valid
value. If the current priority is zero it will get replaced with any remaining
record that has a priority of non-zero, until no records of those exist after
which the records of priority zero are handled.

This change makes it so that the priority of the first remaining record is
the current starting priority. There is also a small optimization to prevent
iterating records when the starting priority is already zero.

Change-Id: I103511f35b50428f770bd4db3ffef70fb6f82d35