domain_codec: optimize dname_dec and convert_dname

dname_dec: now iterates over the packet only once.
convert_dname: remove redundant checks and code shrink.

While testing I've noticed that some of the tests didn't compile
properly, so I fixed them.

function                                             old     new   delta
dname_dec                                            286     267     -19
dname_enc                                            166     143     -23
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-42)             Total: -42 bytes

Signed-off-by: Martin Lewis <martin.lewis.x84@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed