core: prevent invalid socket symlink target dereference (#4895)

core: prevent invalid socket symlink target dereference (#4895)

socket_find_symlink_target() returns a pointer to
p->address.sockaddr.un.sun_path when the first byte is non-zero without
checking that this is AF_UNIX socket.  Since sockaddr is a union this
byte could be non-zero for AF_INET sockets.

Existing callers happen to be safe but is an accident waiting to happen.
Use socket_address_get_path() since it checks for AF_UNIX.