interface: fix value stored to 'fd' is never read

interface: fix value stored to 'fd' is never read

Fixes following issue reported by clang-12 static analyzer:

 mdnsd/interface.c:360:3: warning: Value stored to 'fd' is never read
                 fd = -1;
                 ^    ~~
 mdnsd/interface.c:394:3: warning: Value stored to 'fd' is never read
                 fd = -1;
                 ^    ~~

Original author most likely wanted to assing -1 to the original
interface filedescriptor.

Signed-off-by: Petr Štetiar <ynezz@true.cz>