smart_pkt: treat empty packet lines as error The Git protocol does not specify what should happen in the case of an empty packet line (that is a packet line "0004"). We currently indicate success, but do not return a packet in the case where we hit an empty line. The smart protocol was not prepared to handle such packets in all cases, though, resulting in a `NULL` pointer dereference. Fix the issue by returning an error instead. As such kind of packets is not even specified by upstream, this is the right thing to do.