lm-asyncns-resolver: Fix resolver uninitialization * Hopefully fix crash, when resolver CB gets called on non-existing object, if there's some problem with DNS and library user destroys LM instance before DNS timeout. * This introduces freeze, if underlying libasyncns uses pthreads instead of forking, but that is a bug in libasyncns, that will be hopefully fixed by upstream. Not reported yet.

Frank Zschockelt <lm@freakysoft.de>
Mikael Berthe <mikael@lilotux.net>