build: Fix ast_sockaddr initialization to be more portable

A change to glibc 2.22 changed the order of the sockadddr_storage
members which caused the places where we do an initialization of
ast_sockaddr with '{ { 0, 0, } }' to fail compilation.  Those
initializers (which we shouldn't have been using anyway) have been
replaced with memsets.

Change-Id: Idd1b3b320903d8771bfe221f0b015685de628fa4